Just some thoughts:
An average inventing rate sounds good to me.
Concerning recipes...why not take some rigorous action and instead of throwing them out make the entry fee 10 times cheaper. People are always interested in recipes, whenever new recipes appear in the game people want them, kind of collecting them (I do). And having entry fees from 1-10k makes that those Arenas will move fast. Same like with potions you give newer players a chance of competing for "real" prices and when more people have the recipes....well, they might even think to try to go win some of the parts.
...and yes, partly following all those previous comments and threads...make all arenas with combat moves, they only exist for the arena after all! And put a max of 50% on Arenas with epics allowed.
A suggestion on a combat move tweak: Piercing Strike can "destroy" high Armor when hitting. For high Defense you get Weaken...a chance to decrease Defense with 25%, even if successful you still need to wait a turn before doing damage, and decreasing 8k Defense still leaves 6k Defense

. Crush then has this great, Piercing Strike-like effect of destroying high Defense...only problem is to actually hit...and if you manage you usually win anyway. So, why not combine Weaken and Crush, a kind of Weakening Crush...25% chance to decrease Defense with 75%.