What are the current level caps on each map?

[Royce]

I was wondering, what are the current level caps for each map, and what expansions give you the next cap?

Each campaign says what levels it is for, and you can still gain reduced experience up to 3 levels below, so:
Forest haven (1-10) you can reach 13
Dark Forest (10-15) you can reach 18
Balefort Castle (15-20) you can reach 23
Fathom Keep (20-25) you can reach 28
Lost Expedition (25-30) you can reach 33
Ancient Swamps (30-35) you can only reach 35 for some reason which makes no sense obviously given the 3 level down rule

Also, does buying just the expansion that has the highest level allow you to skip over the other expansions and max out to 35?

Technically yes, but you will run into trouble finding games in the swamps with a level 13. People will call you a leech (which you would essentially be), and boot you from most games. You could probably get away with skipping every other campaign (ie FH->BC->LE or something), but I wouldn't try to go Forest Haven to Ancient Swamps if I were you.