I will try to answer some of your questions as best I can.
1. All the 1200 series have direct wired cables. They are not removable. There may be a way to mod the table and install RCA jacks into the plinth but they do not come from the factory that way.
2. The 1200mk2 (the original) comes with hinges out of the box. The later variants (MK3, MK5, and M5G) do not come with hinges. KAB sells the kit with all the parts needed. There are pre-drilled holes on the plates in the back and can be used to mount the hinges. KAB also includes the replacement plate the MK2 uses but that requires disassembling the TT.
3. I can't speak for the person who got feedback, but I highly doubt it was an issue with dust cover. His setup had a lot of speaker nearby with nothing to isolate the TT. The 1200MK2 series is very good at isolating vibrations but it can only do so much. He posted he was able to resolve the feed back issue by using an additional platform to isolate the LF vibrations.
Personally, I have not had any feedback issues with my M5G yet. At least not at any volume level I can take. Of course the system it is currently hooked to is not exactly a power house.
4. The MK2, MK3, Mk5, and M5G are all based on the same design. They are for all intents and purposes, the same TT. However each later model had some minor revision that enhanced the TT for DJs. Such as dropping he hinges, LED used for the cue light, slip mat included, pitch control made digital, etc.
The M5G has all those revisions in addition to an improved arm and wiring. This is the latest variant of the 1200MK2 series.
How much better does the M5G sound over the previous models? No way to be sure unless you do an A/B comparison using the same cart, record, and equipment. When I bought my M5G, I had to take it on faith the extra money was worth it. It was not that big of a jump in price and so it was not the big of a jump in faith.
The plain old 1200 MK2 is still a fine TT. If there are budget concerns and a M5G is just out of reach, I say buy the 1200MK2 without remorse and enjoy.
Note: 1200 = silver plinth. 1210 = black plinth
(If anyone sees any inaccuracy with my post, please feel free to correct me)