[RPG] Are there any official guidelines for Shadowrun payouts

Conventions are tricky beasts. Ask anyone who has demonstrated a system (new or old) or run a tournament they will tell you to live by the saying "your best laid plans will never survive engagement with the enemy".

The best rule-of-thumb I use for conventions is KISS (Keep It Simple Sammy).
Remember in any presentation what your are trying to do. You have a goal for being at the convention with your stuff. Never lose sight of that. It sounds obvious but the distractions of the convention will try their best to re-direct your focus away from that. Gamers love rabbit trails and will take you down one in a heart beat without even a thank you.

Your list of constraint items is right on the money. You are setting up a very limited, controlled scenario with strictly defined characters and a tight focus on the scenario objective. Do not allow variation, AT ALL, because you are the one who has to adapt to any changes on the fly and you (and your game) will be the one who suffers if there is confusion.

There will be changes and plenty of confusion.

Here are the 'rules' (guidelines) to setting up your demonstration for your new RPG system.

1. Have a script. Not for the scenes or for the demo but for you. Nothing elaborate or written out but something well defined in your mind. You should know pretty much want you want to say when someone approaches you and it should be short. People who are well spoken tend to draw an audience and those who stammer through a presentation soon find themselves with no one to talk to.
2. Teach. Do not try to play the game first, teach the game first. With everything you use whether it is the pre-gens, the scenario or handouts you want people to take with them should be something that teaches a part of the system. Even if this means the scene doesn't make complete sense. You can tell the players that you are doing this to teach and they might want to do something different. They will use this a way to say I can do this.
3. Fast. No one at convention comes to play slow, drawn out games. Your 4 scenes' should be able to be done quickly. You define what quickly means based on your system.
4. Flash and Flair. Everyone wants to be the hero, so setup each scene to highlight a specific character or 2 and let them shine. "YOU GOT THE VICTORY CONDITION" will promote a game faster than the slickest system out there.
5. Leave them wanting more. P.T. Barnum always left his customers with the hint of something more to come. While it sounds crass it is the number one marketing tool you have. If they like what they see (and not everyone will) then they need to feel there is something more to be gotten if they come back (purchase your system).
6. AT ALL COSTS AVOID -- Arguing with the gamers. There will be a person who has done this already and will tell you their reasons why you are wrong. Thank them politely and repeat back to them what they said so they know you heard them. If you do this they go away quickly and leave you to run your show. Do not be afraid to say to someone 'Please wait, while I am in the middle of this.'
7. Have something to give to the people that reminds them of your system. Do not do a cheesy quick flier, give it some thought and make it usable. Maybe a quick start rules or copies of the scenario. Nothing draws people faster than getting something that has real value or usability. Have plenty on hand.
8. Always look like you are having FUN and you are EXCITED. Do not be crazy, just happy to be there and ready to talk or game.
9. Do a number of test runs before the convention at your local FLGS or with your group and time your self will doing it. This will hone the scenario and your talking points.
10. Finally - have something that you can do in 2 minutes to lets your system shine. Just the most basic bare-bones of a scene with a single character. This lets you build an audience and get a few folks interested in the long demo.

Unfortunately, we won't see these guidelines (unless part of them show up in the previews), until the DMG.

So I can state confidently, that these rules do not exist in official published materials.

For now that treasure generator looks like a pretty solid option.

With respect to creating your own shops, unfortunately, the economy is based around magic items being rare enough that they are incredibly hard to both purchase and sell, so I'm not sure we'll see a list of shop inventories in 5e. The best I can do there is that the starter set has some guidelines on what mundane items shops in Phandalin have.

With respect to recommended items per level, again this is advice we simply don't have. But it's worth noting that in general items we've seen don't have levels. That doesn't necessarily means they wouldn't be overpowered for certain characters and underpowered for others, it just means we haven't seen levels attached to them. We have seen +1 attached to weapons and armor, so certainly you wouldn't want to give a L1 character a +2 or +3 weapon/armor, but beyond that the guidelines are non-exisistant at this time.