Do wolves and slimes even exist any more as I haven't seen either since 1.7 update.
I have seen both slimes and wolves in 1.8 since the release. Because mobs are more persistant it may be harder to find some of them than usual. I saw slimes in SMP and wolves in a newly spawned world in SSP. You just need to keep looking. They exist.
Well, they aren't fully persistent! The animal spawner does attempt to run every four hundred ticks, and if it can find places to spawn, it will. (provided that there aren't fifteen or more animals loaded at the moment) As a result, new spawns are entirely possible, but rather rare.
BTW, in case you do not already know Abandoned mines are not the only sources for pumpkin and melon seeds. If you have a fully grown stalk of either watermelon or pumpkin and you break it, you'll get 0-3 seeds back. You do have to start at minumium with one seed that you found in an Abandoned mine, but at least you don't have to find ALL of your seeds there. This is very helpful with pumpkin farming, at least so I have found.