From my view point, the fairer explanation for Leia/Chewie actions are those:
1)Leia had her mind focused on Han, but killing Jabba would be her side objective as well (though it dont seem like the rebellion methods).
2)Chewie part, fortunately, is more comprehensible. Han in or out of his carbonite prison, was defenseless. He was, since the beggining, the guy that would protect and oversse him. He was also the best option for Leia have a reason of showing up, disguised as a bounty hunter.
I'll have to admit that its a very poor plan, anyway.