Depends on the cave profile, there are a few specific caves that get really shallow a long way back. If it's steep up then right back down to where you were you have the option of either decompressing on the mount or hammering the go-fast button and going full tilt boogie up and back down the mound to get back to depth before you get bent. If it doesn't go back down to your ceiling depth then you have to do decompression back there. It's not fun but it's only a few specific caves that have that type of profile, very rare
Yes, it depends on the cave. I have done a dive in a cave where it goes to 75m at 900m from the entrance. This can be done swimming, but it goes to 2m, then it drops suddenly to 75m for 180m. If you do it swimming, you have to return, wait till deco is gone before you cross the 2m. To do the first sump, you need a dpv, to do partly the sump, you can do it with a swim and return after some minutes at 75m depth. So if you do the sump by dpv, you also do the deco far in the cave of course. Saint Georges cave.
In another cave, the first of +/-700m is really shallow, then it goes to 75m, later you can go over 100m depth. Here you can do deco partly swimming. I talk about the Tannerie. Also here a french guy is still exploring, and he uses a dpv because he goes far and deep (I believe he has explored this cave to depth over 275m, and these are solodives, really extreme. But it is interesting to follow).
The goul du Pont reaches 120m depth at 210m from the entrance. So you do deco just waiting at one place, not swimming, no dpv needed. But if you want to go far and deeper, then you need it. There is a French guy, I mentioned already about the TAnnerie, that dives almost every day there because this cave is in his backyard. He uses double ccr and dpv's. But he goes to over 200m quite often. He is still exploring here. Some people believe the Tannerie and the Pont are connected, both entrances are only 50m away from each other. But were the connection is? Normal dives will never see this connection.
To reach the 100m in the Saint Sauveur, you need a dpv, it is about 450m from the entrance, but the cave drops directly to 70-74m. The deco is done at the exit of the cave.
To reach 100m depth in Font Estramar, you also need a dpv, that is also a 1000-1200m from the entrance, but a long time you are at 38m depth.
So it depends of the cave. That means for some people or dives a dpv cave cert and a ccr cave cert and a trimix cert?