There's not much you can do about the time. It's Munchkin. it's a 30 minute game that extends to two hours with the regular number of players. More players means more piling on. Munchkin takes so long because you win not by defeating enemies but by out lasting the counter blows from your other players. More people mean you have to be even stronger to break through. The best you can do to address that is to start the pile-ons sooner.
1. Change the goal from 10 levels to 5.
Muchkin is a 2 hour game with four people. More people means more time. With level 10 as the goal people wait till you hit level 8 or so before attacking you. With 10 players that's going to take forever so set the goal to five. The beginning will be odd but people will push those attacks faster and you can begin the end game war of attrition a lot sooner.
2. Reduce monster values
Because the goal is level 10, monsters can be worth 2-3 levels because it generates a fun swing when you gain a few levels and then lose a few levels. With this many people. that swing is what makes the game so long. Instead make every monster count. Each monster is worth 1 level for every say 3 levels out there. If you would normally gain 3 levels.. too bad it's just 1. If you would normally gain 4 levels. Gain two.
3. Buy expansions
Honestly even with 12 people hoarding items card limitations is a very easy problem to fix. First off remind people they can sell items for levels. There's a lot to be gained from selling levels instead of jumping into the firing squad. But if all else fails there are a TON of munchkin expansions that are cheap.
4. Limit Bags
In the rules your bag is unlimited. You can carry whatever you want and are only limited in what you can equip. Instead set a firm rule. Maybe you can carry everything you can wear, one alternative. (so for instance, One head, one arm, one arm, one food, one body, and for an alt a two handed OR an extra foot). And then in terms of everything else no more than say 5 items. This could also encourage people to sell stuff rather than hold on to it. To use those attack rather than to wait for someone to get to level 4.
These limits should get people more invested in attacking, more willing to attack, make attacks more serious, and make them more manageable as people won't be able to hoard 20 levels of monsters until you get to level 9 causing you to lose 4 levels or die. It makes the game harder to keep going. Thus ending it sooner. Card turnover isn't really an issue in my opinion. Cards aren't so powerful that seeing them again and again is really something to be concerned with. With the exception of removing every "Gain a Level" card from my deck (because I DID see those a lot and they're not fun/interesting) I've played many games and never seen a card I would not like to see again. The best I can do to address the balance is to limit the weapons people can have. If everyone blows their wad on you the next guy won't have anyone doing anything. Now people won't have the gear to attack everyone every turn. So hopefully this means that some of them are always gaining gear and some of them are always using it and not everyone at the same time is using it. Ideally on any given turn you won't have 7-10 enemies because there wont' be 7-10 people with that much gear because they'll be using it on people before you.