Reach Spell it is then ...
Edit: Complete Divine states it's actually two spell level slots increase.
Sorry, 2 levels it is, I'm remembering stuff wrongly.
Can I craft my items in general? Can I use the reducers in DMG for requiring a specific class or skill (I think that's 10% each)?
Yes, the others in your party wont be able to use those items though, unless they succeed on an UMD check
Can I make custom magic items (using the DMG table)? Basically I want an item that provides a bonus to my UMD checks and something that lets me cast unfettered heroism once or twice a day.
You can make custom magic items, but I want to say yay or nay on a case by case basis, I don't want continious true strike rings, or lesser vigor amulets, which are ridicolously cheap yet are much better than almost anything else out there. It is a yes, but be reasonable with the items.
Can I use the experience from my previous craft reserves? Of course, always appropriate to the levels (for example, I can use the first level's experience cache only for crafting scrolls up to 3 caster level).
Sure, go ahead with that, it makes sense your character has crafted to get to level 12
Can I use magical artisan (PGtF), extraordinary artisan, legendary artisan together? If yes, what's the reduction?
Yes you can.
0.75 * 0.75 is 0.565, so you would pay 56.5% in the respective area.