These appear to be developer questions, so might be better on the developer mailing list. Even then, I wouldn’t rely on getting authoratative answers to all of them, as they look like pre-sales questions and there is no pre-sales organisation for the open source code.
Assuming you want release versions:
Go to svn.digium.com/svn/asterisk/tags. The highest numbered 1.8 and 11 versions, without suffixes are the two most recent released source code versions. 11 has more features, but 1.8 will be supported for longer.
If you want leading edge versions, that is:
There are also branch versions for 1.8 and 11.
I don’t know. It is possible you will have to try. Some of the proprietary codecs are only available as binary. The easiest way is to try.
Asterisk does not contain any GUI. The GUIs are separate products. The source code for the GUIs is available, but I’ve never had cause to use it.
I don’t know. It uses the ./configure mechanism, so will try to cope with lots of platforms, but may not have been tried on yours.
Only patches that are considered safe, originate from Digium or have been submitted through the issue tracker, with proper licensing, and don’t constitute new features, are in the release source code. The trunk source code will contain new features and may contain patches that are not considered safe for release.