I figured I'd try to answer the part of the question that most of the other answers haven't really touched on yet, namely "How do I decide whether a particular piece of tech would be too disruptive?"
One way to approach this question is to ask yourself, "How could the players achieve the same effect using things that are listed in the books?" If all the effects of the tech you're considering could already be achieved by other means (even if they might not be quite as practical), it's unlikely to completely break the setting. It might still have an effect on game balance, of course, but you'll at least have some kind of upper bound on how much difference the new tech could make by comparing the difference between the new and the established ways of doing things.
Taking your portable medkit for synthmorphs as an example, you should ask yourself "How else could the players heal a sick or injured synthmorph?" Maybe they can take him/her/it to a hospital; a hospital isn't portable, but it means that all the players are saving by using the medkit is the time and effort to travel to the nearest one. That could still make a big difference, depending on just how far the closest hospital is, but at least it gives you some idea of how big the difference could be. Or maybe your setting has magic as well as tech, and there's already a healing spell that works on synthmorphs, in which case having the medkit just means the players don't necessarily need to bring a mage with healing skills along.
Of course, when using this method, you should generally try to keep the effects of the new tech fairly conservative compared to what already exists in the game. For example, the medkit probably shouldn't be able to fix anything the hospital couldn't. In fact, its abilities should most likely be strictly inferior by a considerable margin, both for the sake of realism and to offset its convenience. Also, since the players can't really keep running to a hospital every few minutes, there should probably also be a limit on how often the medkit can be used, and/or on how long it takes to do its job. And, of course, if the setting clearly implies that something is generally lethal or disabling to synthmorphs, well, the medkit probably shouldn't be able to fix that.
This conveniently segues into another question you can ask yourself, namely "What effects would the existence of this tech have on established (or planned) elements of the setting?" For instance, try to think of any events in the backstory of the setting where having this tech might've made a difference. If you find yourself thinking "Well, those guys sure wouldn't have lost that battle if they'd had these medkits," and you can't think of any good reason why they shouldn't have had them if they existed, that could be an indication that adding that tech to the setting might at least disrupt the consistency of the backstory, if nothing else.
Also, think not just about what your players could or would do with the tech, but also about what their enemies (and other NPCs) could do with it. You're going to need to think about that sooner or later anyway, unless there's some good reason why only the players should have it, so you might as well think about it before you decide that it really exists.
If you have a suitably devious mind, you should also ask yourself "How could I abuse this tech if I wanted to?" Obviously, that means not restricting yourself to how you or the players think the tech should be used, but just looking at the stats as you've written them and thinking "OK, if I wanted to min-max and exploit the hell out of this thing, and had enough resources to pull it off, what could I do?" OK, so one portable medkit seems pretty harmless; what if you had a hundred of them, and combined them with all the most exploity features already in the game, what could you do then?
In particular, beware of anything without limits. A box that can unfold to twice its size is probably harmless. A box that can keep exponentially unfolding forever could easily break the game.
If you're not feeling so devious, a simpler alternative can be to just ask your players up front what they want to do with the tech. Then, if it sounds reasonable to you, write the specs so that it does that and nothing (too much) more. One advantage of this method is that you can harness your players' creativity in finding ways in which the new tech could be troublesome. (In particular, the kind of people who like to come up with exploits for game mechanics are also typically happy to point out those exploits before the mechanics are made part of the game, just as long as they get the chance to demonstrate their cleverness.) It also, in effect, binds the players into an unspoken promise not to step significantly beyond the limits they themselves set, or at least gives you and excuse to step in and say "Hey, wait a minute, that wasn't the way it was supposed to work!" if they do.
Finally, if you're not sure, don't be afraid of saying "OK, let's try it and see." Playtesting is an essential part of balancing the rules of any game, and there's no reason why you can't do that with your house rules too. Just tell your players that they can have the tech for one session to begin with, but that, if it unbalances the game too much, it's going to go away (or be redesigned) for the next session. In extreme cases, you can even tell your players "OK, that was just way too broken, let's just start over and replay the session without it." As long as the players knew in advance that this was going to be a test session, and as long as they can agree that the test didn't work out the way it was supposed to, they'll understand that.
For me this is largely about expectations, signposting and player agency.
The first point is expectations. How much was the tone of the campaign discussed before you started? Did you make sure the players were aware you were going for a world where their actions can have negative consequences and that you would follow through on these? This might sound silly, as I'm sure for a lot of people what I've just described is where the fun of roleplaying is, but you'd be suprised how many players have the expectation that they are never going to fail.
Then there are signposting, opportunies to spot the bad stuff and if it happens then do something about it. If I were a player and the only warning I got that something really bad was going to happen was right at the beginning of an adventure 3 months ago, then I think I would be entitled to be a bit frustrated. People have short memories and cannot be expected to be able to identify the important bits of information in everything you tell them as GM, let alone remember it for weeks/months at a time.
Lets take your first point as an example. Have they been made aware that the Kobolds renege on deals and have a reputation for not giving gold? When was this and by whom? Was it someone they had a reason to trust? If their only warning was a brief encounter with a minor NPC 10 weeks ago then you might want to think about reinforcing this somehow.
Assuming that you've given them plenty of warning and they still end up with no gold, the most important thing is to ensure that you provide a way of getting revenge and/or recompense. I know if I'd been doing a load of dangerous work and didn't get the gold I was promised that I would want to do something about it. The most important thing here is to give players the opportunity to act in revenge/response to whatever has happened to them. Maintaining player agency when something bad has happened to their characters is key in ensuring they don't feel hard done by or upset. There is nothing quite so bad as effectively being told 'Haha! You don't get any of the money you were promised and there's nothing you can do about it!'
Best Answer
There are many ways to give treasure to players that you can activate/deactivate depending on how the players overcome the encounter. If you tie the encounters to the story, and if you tie the equipment to the story, there are plenty of opportunities for reward other than looting.
Make random encounters not quite random - For example, instead of stumbling upon 2d4 goblins in the forest, stumble upon goblins attacking a caravan. If the players kill the goblins, they can loot them, if the players scare off the goblins through some elaborate illusions and successful bluff checks, they either get gold from the caravan, or they get to guide the caravan to the next town where the local chief is so overjoyed to see his daughter safe and sound that he gives out a sword of +2 killing.
Put the treasure next to the enemies, not onto the enemies - I'm not particularly fond of "I loot a wolf and I find a sword of +2 killing". How does this even make sense? Also, why would a band of goblins carry all their spoils from previous raids with them? Much more likely, the goblins have stashed their gold at their camp, and if the players scatter the enemy, they will later stumble upon the abandoned goblin camp, where the goblins forgot a chest in their haste.
Hand out the equipment up-front Say that the players have successfully saved that caravan. Now they're hired as bodyguards, because obviously their method works. However, the caravan leader notices their shabby gear, and fears that the PCs will not be strong enough to guide the caravan through the valley of certain doom. So they equip the players with new shiny gear that they can keep if they do a good job.
In response to the specific examples in the question Assume I'm a GM desperate to give treasure to the PCs (why would that ever happen, srsly); how would I deal with the four examples you provide?
You take the shortcut, you find the treasure, which has been magically augmented by the treasure you were supposed to get from all the enemies (note: in my games, you would most likely not have gotten much treasure from the enemies, anyway)
The gnolls leave, and when you look through the remnants of their dinner (some unlucky kobolds) they were having while waiting for you to leave, you find a hidden pouch in the discarded leather armor of the kobold leader, containing $treasure.
The goblins take a liking to the hobbit, and point him to a bag of loot they carry with them; mostly farm equipment, but also a rusted holy axe of smiting (how did a "villager" get that?). They tell you guys to help yourselves, but that you'll own them a favor.
Either somebody is glad the living spell is gone and gives you stuff for it, or, as you close the vault door, you also hear some kind of rattle, only to find out that part of the treasure is hidden inside the vault door.