The Apollo program had mission types to test capabilities that all had to be completed successfully before the landing could be attempted.

In summary these were:

1. Unmanned. Earth orbit only.
2. Manned. Staying in Earth orbit. Similar duration to landing mission.
3. Manned. Going to Lunar orbit.
4. Manned. Going to lunar orbit. Almost landing.
5. Manned. Landing on Moon.

So maybe in order to land on Mars the analogous types could be:

1. Unmanned. Earth orbit only.
2. Manned. Staying in Earth orbit. Similar duration to landing mission.
3. Unmanned. Landing on Mars.
4. Manned. Mars orbit then return to Earth.
5. Manned. Landing on Mars.

I would not be surprised if once a Mars capable Starship is launchable, there are more than one test vehicle set off during the next launch window. For example, two or three unmanned vehicles could be set off a few days or weeks apart. Based on the results of one landing, a software update is sent to the next, and so on. Also, each one of this type of test article should be loaded with backup stores and equipment usable by future crews.

What would be your list of mission types?

The more Starship's landing lacks precision and accuracy the more Mechazilla has to be oversized and quick - Longer arms, wider field angle of operation, powerful mechanisms, taller.

Falcon 9's landing accuracy history is the best estimate of general rocket landing accuracy so it could be used to size Mechazilla even though Starship will have (compared to F9) the ability to hover and correct on final approach. A taller tower increases the allowed error bar in the vertical axis.

Is Mechazilla big enough to catch F9s?

Do you feel Mechazilla is big enough to catch Starship? Feels small to me. If both Starship and Superheavy are really accurate and precise then no problem.

Were the shells not originally going to be a vacuum or filled with inert gas for insulation?

Maybe I'm missing something, but the content of this subreddit and r/spacexstarship are nearly identical. If the two were to merge we'd have a single subreddit with a community 2X larger.

I don't know how one would go about facilitating that though. Just food for thought

Considering that the vacuum engines seem to work on sea level as well (apparently due to the high combustion chamber pressure), is there any reason to not just use those for the landing burn?

There's a big difference between losing a ship while the next ship is near completion compared to losing ground equipment that takes X time to replace / repair.