Some years ago, the great toysdream wrote a related thread for the now-closed Gundam Evolution forum, which partially answers to your questions.
Here's an excerpt of it.
Posted: Dec 24 2006, 01:43 AM
I was just looking at Model Graphix's "Gundam Wars II: Mission ZZ" book, and I noticed that there's a really fascinating four-page section in the middle of the book which discusses the planning process for the mobile suits that appear in the second half of the show...
First, let's back up to the beginning. Gundam ZZ had an unusually complicated planning process, full of last-minute rewrites and staff changes.
The original plan for the second half of the series was that the Axis forces would stage an invasion of Earth using ballutes and HRSL capsules, landing a large force of mobile suits without any motherships or support vehicles. Designs were commissioned for a variety of amphibious mobile suits - the Zaku Mariner, the Gaza Mariner, the Methuss Mariner - which both sides would use to secure their splashdown sites and escort their landing forces safely to dry land. The mobile suit forces would then have to make their way over land in caravan formation; the Mission ZZ book notes that it's impossible to operate flying platforms like the Dodai and Base Jabber without a support base on the ground, so instead we'd have a lot of a ground convoys centered around vehicles like the Zaku Tanker.
At this point, the plan was that the Axis landing forces would consist largely of upgraded One Year War-era mobile suits. Specifically, these would be space mobile suits retrofitted for ground combat; for example, the Dowadge was originally supposed to be a ground version of the Pezun Dowadge, which is why it has the same name, and the original design for the Desert Zaku was based on the MS-06R High Mobility Type rather than the One Year War-era MS-06D Zaku Desert Type. Many of these Axis mobile suits, such as the Zaku Mariner, were meant to be variations of a new "final model" of the Zaku series known as the Zaku III. However, since the Zaku III itself hadn't been designed yet, the designers just assumed it would look pretty much like the standard Zaku II and figured they could match up the details later.
Then, at the last minute, the writers changed their plans and decided to just give all the spaceships Minovsky craft systems so that they could carry their mobile suits down to Earth. This eliminated any need for ballutes and HRSL capsules, not to mention amphibious mobile suits to secure the landing sites. And since the Axis ships would be loaded with the same mobile suits that had already appeared in the first half of the show, all the Dom and Zaku variants that were in the design process suddenly became leftover One Year War machines operated by Zeon remnants who had stayed behind on Earth. (Any postwar technology they possessed, like the huge beam cannon carried by the Dowadge Kai, was explained away as booty plundered from Federation bases.)
The Zeta Plus, which was meant to appear during the Dakar battle, was eliminated for other reasons. Since there was already a Gundam team aboard the Argama, the writers were worried that adding even more Gundams into the mix would simply confuse the viewers. They had also decided to push the rest of the AEUG and Karaba forces into the background, which is why the Sturm Dias, originally meant for use by elite AEUG special forces, ended up being given to Zeon loyalists instead. In the end, the only new AEUG/Karaba machine to appear in the story was the GM III.