1. Unknown. No one has been able to get into space or establish contact to verify. You could assume either way but most people would probably think it is a planet.
2. We're using Dynasty Warriors gundam as the base for the roleplay. The End mission for Heero Yuy/Jerid Messa/Master Asia is the Twilight Citadel. This is from the first game. The military complex area is a standard military base but if you want more visuals than it can be seen here:
https://www.youtube.com/watch?v=HvJGrt5XvzQ&list=PLDK9RMYEqcFOei07jkvgVkkCRSxXLIfzU&index=6It's a let's play so you may want to mute the voiceovers if you don't care for it and it's not exactly meant to show off the locations. However there isn't exactly a reliable way of getting a full on description of a dynasty warriors map.
Location wise they're a few hours away from one another. There is no general geographical map.
3. The ZAFT city is a target because it is currently projecting a signal, and is a known enemy of the current heavily biased OMNI Enforcer faction that is allied with Mariemaia (Jason's entire crew is OMNI). As such it's natural to go after a known enemy. Besides that there could be supplies and other useful things there. At the very least we can get out of the desert.
4. Mariemaia's Army is not the same army from the sense of Endless Waltz. This Mariemaia won her battle and has settled down on Earth becoming World Leader like her father. She's smarter than the original and thus threw Dekim to the dogs once he proved useless. Most of her soldiers are loyalists from her own regime however Kenta from OMNI has taken over tactical strategies for her own troops and has a very powerful suit which has yet to be revealed.
Likewise everyone is stranded with no chance of contact so alliances for better or worse have been made with many of the gundam characters that normally wouldn't be with each other. It has a similar outlook in the DW games where people like the above mentioned characters and others like Trowa Barton, Banagher Links, and Ribbons Almark are all a part of the same faction.
What they stand to gain from this exactly is more military power, and a combination of resources and information to help everyone get home. Likewise both of them have enemies in Zeon at this point in time.
5. I haven't given them a finite number. I'd say around twenty or so Serpents, and 5 of Kenta's Daggers whom were originally a part of his now missing task force.
6. Just look back a page or two in the IC you should find it if you just look for 'Report' under control F or search for it with the search function on the site. It's just a large list of everything Jason has encountered.