[RPG] Should drawing weapons involve an initiative roll when combat isn’t going to happen

Encouraging inventiveness is easy. You've already done it, in fact: You simply have to reward it by allowing it to (occasionally) succeed.

Avoiding breaking the game is also easy: You simply disallow anything that lets players to bypass the obstacles presented by the game with less effort than it would take to tackle those obstacles in a more traditional way.

Obviously, these two goals are hard to reconcile, but I can suggest a few things that might help.

First, let your players have the benefit of their invention, but remember to account for the costs: In your example, your alchemist has developed a devastating weapon, but one which eats through very expensive ammunition at a disturbingly rapid rate. Once the disparity between party member wealth levels grows a bit, he might start to worry. Also, given that it is a devastating weapon, even if it is expensive, ask yourself why other people don't use it. I imagine it would take just one orc with the Improved Sunder feat to make your player realise that there are pitfalls involved in waving acid vials over his own head.

Second, remember to include at least a few situations in which the invention does not work, to keep your players on their toes (and encourage future inventions): There's no end of creatures immune to acid and fire, so let them crop up occasionally - and your alchemist would be in strife if away from civilisation for an extended period.

Finally, don't be afraid to condtradict previous rulings if you can come up with a good reason for it. Yes, you should avoid it most of the time, but inventions are a special case, since by definition they're trying something new and unknown: Invention is inextricably paired with discovery. Be tactful, and start by saying "There's something I didn't think of that's kind of relevant to that idea you came up with," but be firm, and try and provide a reasonable in-game justification for it. "Your acid net? The ropes that make it up only have about two hit points, so it should be disintegrating every time you use it. Also, how have you been protecting yourself from the splash?"

Try to ignore segments. They're a concept that was mostly introduced to keep track of how long it takes to cast a spell (more on that below), and aren't helpful for the rest of combat.

Instead, concentrate in the actual initiative results. Treat the action of a round as mostly simultaneous, but with the winners of the initiative getting the advantage. So it's not really that the losing side stands still for 30s, it's that they both maneuver, but the side with the initiative simply maneuvered better and got the battle line they wanted.

So in general, just use initiative to decide what order to resolve actions in. You can layer on house rules on this to make it work more how you think it should, but it's not necessary. (One house rule you'll see sometimes is resolving all combatants' movement in initiative order first, and then resolving actions in initiative order, to make the feeling of everyone just standing around go away.)

As for spells, a spellcaster already in mid-spell from an earlier round doesn't act on the initiative number of their side. Segments are counted to find out when the spell finishes, and until it does they are occupied. This often won't matter, as many spells are fast enough to begin and finish before the enemy acts or the round ends, but when the initiative rolls are close, then it matters, and when the spell takes a lot of segments to cast, it matters more. The most important thing segments tell you is: does the magic-user get hit before the spell finishes, disrupting it? and, does the spell take up their next round as well? Segments exist almost entirely as a tool to determine the answer to those two questions.