If I understand the current Lunar errata spoilers, a Lunar should be able to, at essence 10, grow legs long enough to cross the entire distance in one step.
Other than that, or some sort of perfect racing charm like the Unconquered Sun has or the Yellow Path suggestion I made, any eclipse-like character taking all the speed boosting charms they can get should be able to, like you originally said, have the record.
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?"
Best Answer
There is no good way to have what the book calls a scimitar that is a light weapon.1 Burning feats or taking −2 attack penalties for wielding an undersized scimitar are ridiculously painful for something that should be very simple.Instead, take the stats of a kukri, call it a “light scimitar,” and you’re good to go.2 The rules of the game never define that the game-item called “kukri” has to look like the real-world weapon by that name.3 It’s just a bundle of stats with a label so it can be conveniently referred to in the rules and can be balanced against other weapons. How your equipment looks, as an extension of how your character looks, is really up to you. And what your character decides to call that item is definitely up to you.
And a kukri is the light version of the scimitar: slashing damage with 18-20/×2 critical threat range. The damage die is slightly smaller (on average, 1 damage point), but aside from this, the weapons are nigh-identical. Losing 1 damage is a lot less costly than any other option available to you.
And there is no good reason why you should have to waste resources on getting to describe your character the way you wish to.
I was mistaken; @Barillas found a pretty solid one in the form of effortless lace, which should cover the requirements for most purposes. Not an intrinsic part of the character, but there don’t appear to be any of those.
This may fail if you have particular feats or class features for what the book calls a scimitar – if you were given a choice, you can always choose what you call a kukri and I call a light scimitar for feats like Weapon Focus (and thus may want to wield two of these light scimitars, rather than a scimitar and a light scimitar), but other options may not give you a choice. In this case, you really do need what the book calls a scimitar unless your DM agrees to change it. I feel that in almost-all cases, a reasonable DM should have no problem with this, but that’s a conversation you’ll have to have.
Though DMs do probably have to draw the line somewhere; saying your kukri looks like, and is called, a light warhammer or something, probably isn’t going to fly, since something that looks like a warhammer shouldn’t be dealing slashing damage. But saying your “kukri” looks like a lighter scimitar is entirely reasonable, seeing as the two weapons are so similar in the first place.