Just successfully smashed the Ottomans with a large coalition. Even out numbered three to one they still won the majority of battles, what with their 115% discipline, better technology, and morale out the wizoo thanks to piety and prestige, plus poor tactics on the part of my AI allies, but we managed to wear them down eventually. It cost me 133 DIP power to make return a bunch of Hungarian cores though. It'd be nice if my fellow coalition members could have shared in some of that cost; it's kind of a disincentive to be the one to start a war against an agressive power.
No, just... no. It's already debilitating enough to be a junior partner in a coalition war. You can't declare peace, you don't get anything from it other than stopping the enemy, your war exhaustion can get arbitrarily high and get tons of revolts and you can't do shit about it, etc., and you could be the top nation of the pack but you're still a junior partner if any of the little guys starts some crap. In fact, I think it's only a matter of time until a common strategy is to enter a coalition with an enemy, DoW as soon as current treaties end, bleed your fellow coalition members dry, declare peace and then DoW each of the other members in turn now that they are husks of a nation with no soldiers, no money, no war enthusiasm and nothing to offer to other countries for an alliance. That is, unless it's already a thing. It's one of my biggest criticisms of coalitions, although the counterargument is obviously "If you don't like it, don't join it!"
Personally I tweaked the whole dip cost for peace thing. I don't understand why doing something like you explained would cost 1/4 of a tech level. It makes no practical sense why peace should require monarch points. There's nothing sillier than having to wait 3 years just to be able to ask for the peace you want.
I think it's because the peace he asked for was war more than what the CB allowed, meaning you have to have a lot more political sway to force it out of them. Alternatively, it's a penalty for asking so much. Unless that was actually a part of the CB, then I have no idea.