"Maniac" is not so common a word these days, although we all know it. I had a check online (yes, you can do this too), and it seems that it's uncommon to use gerunds after "a maniac for".
I suggest looking for real examples online and working out how we use the word from that. Your examples are understandable, but not natural, sorry.
For the second question, the meaning of "well" changes.
"I can well understand that" = I can certainly understand that. You're right, this is a colloquial (spoken) form.
"I can understand that well" = My ability to understand that is good.