They may already be published, so you can use the already published ones on battle.net. Just be aware that according to sc2arcade.com it looks like these published dependencies are anywhere from 1-5 years old, so it might cause some bugs if you use them since they are likely older versions.
As long as you give credits to the original team then you should be fine.
You're most likely getting that error because you are referencing local dependency files instead of ones on battle.net/the arcade. You'll have to switch them out for SC:MR dependencies published on battle.net (you may have to publish them yourself, as I think the only ones on battle.net are published by other people not affiliated with the SC:MR team, and so they may be outdated)
You have probably accidentally searched for something in the table mode of the data editor. Go back to table view (CTRL + 1 by default in legacy keyset), and remove whatever you have searched for, then go back to the wizard view (CTRL + 2 or CTRL + 4)
It could be an issue related to the footprint of the rock structure. I believe there are placement requirements in the footprint, and if you are reusing a doodad footprint (since its a rock) the placement part may not be in there.
Also if you have questions about Mapping/Modding in SC2 I would suggest joining the SC2 Mapster's Discord, as it has more active users than the forums who might be able to help: https://discord.gg/rbnHxRRuZe
Yes, it can probably be done. SC2 supports both offine, and now with patch 5.0 online map-to-map loading. You would have to utilize bank files a lot to get this system to work correctly. It has been a while since I've played the Rexar Campaign, so I think loading the maps all works similarly.
For example, say you kill a pre-placed mob on map 1, then go to map 2, then back to map 1, the pre-placed mob would return unless you specified that it died via bank data. Also the fog of war would reset each time you load into a new map.
Offline map loading trigger: "Set Next Map" action
Online map loading trigger: "Online Map To Map Load" action