Mango is developed using Eclipse. The best information on the internal structure of Mango can be found in these discussion boards or in the code itself, but that might not be enough for a very clear understanding. If you have questions, feel free to post them here.
That should work fine. Note that the "$("commentText").select();" line is supposed to select the text such that typing anything would overwrite the previous comment, but it would still be there in case it was useful. That line and the following line (focus) should probably be reversed.
No, the new-version check is automatic - there's no interface feature to disable it. You can have it check for beta, release candidate, or released code, but it will still make a connection to check in any case.
Do you run Mango in a private network? is there a reason why it cannot get to the internet?