How does character progression work in Dungeon Defenders?
According to the wiki, yes, you can respec your heroes.
Based on this leveling guide, you can get 5 skills to 50, and get a 6th to 49 points for a total of 299 points. Thus, you will never be able to max out all your skills.
The maximum per skill is 70.
The maximum number of points you can put into a skill is actually 120 (it used to be 100, and lower before that).
The total number of points a character has (was) 299 at level 70, so you couldn't max every skill.