I want my players to be able to edit their bases. Please correct me where applicable. 

I have set to true...
TC
Entity
Teammates

The most common complaint I've heard is that a teammate wasn't able to undo/fix a misplacement from another teammate, and said teammate was offline. 

Setting Teammates to true fixed this. 

But, I have a few situational questions. Assuming TC and Entity set to true, if they built a base but later lose TC control, they should be unable to break walls, right?  I want to prevent them simply using /remove to break back into their base to retrieve anything left if someone takes over their base. 

Another question, an enterprising player on my server figured out that they don't need doors at all, since they can /remove a wall and replace it (assuming materials). I can't decide if this is something I want to prevent or allow them their cleverity and ignore it. This works especially well in Cave bases. A single armored wall is all you find when you head down the tunnel. Thoughts? 

For the first question: that's correct
For the second question: see this

I appreciate that option. If I may ask then, I assume that's in seconds? Also, that's seconds counting from when the when the plugin is loaded (either a server restart, or when I oxide.reload it), right?

If that's the case, 24 hours would be 86400 seconds and that's what I should put if I want a 24 hour remove like the other poster noted? 

Yeah, set it to 86400. 

"RemoveType - Normal - Entity Spawned Time Limit - Cannot be removed when entity spawned time more than it": 86400.0​

But 84600 seconds is too long. lol

Did I misunderstand the intent and purpose of this option? 

You didn't misunderstand it

I still feel like I missed something. Why would 24 hours be too long? 

nope. It's up to you...