2. Yes - to the extent that ancestry's database has the original document. There are inherent problems that are common to having the data into your ancestry tree. One problem is only referencing one page of a multi-page document. Examples include passport apps and where a family is on bottom of one page and top of the next. Another problem is where the ancestry database itself is an electronic database with no paper docs behind it - like the SSDI, or many death records.
3 Yes. FTM is integrated with ancestry. It could always be better, but is quite good and can transfer the data into Facts with Sources, often into multiple people in your file with simply "discard" or "keep" decisions. This is one of the main reasons to use FTM with ancestry, because you will not be able to access documents in ancestry trees when you stop paying your subscrtption.
4. FTM has always been forward progressable. There was one change that was not "backwards" regressable - that was when the old FTM program, FTM16 and earlier was completely rewirtten into a new computer code and database structure about 2006 or so.
5. There is only one version of FTM for Windows and one version for the Mac.
6. We'd all like to know the answer to that one. People have reasons for preferring the Master Genealogist, Legacy, Roots Magic and others. In spite of its weaknesses, I believe the overriding reason for using it is its tight integration with the ancestry databases.