Let' start with the end: You generally need a new LNP for a new DF version. DFHack is tied to a specific DF version and there are checks against trying to use it against other version (all offsets would be wrong, so nothing would work and the game would probably crash if that safeguard didn't exist). In addition to that, there is some work going into making sure the various other tools in the LNP are compatible with the DF and DFHack versions.
DT is a different beast, because it uses a file to find how to connect to DF, so it often is compatible with several DF and DFHack versions.
Short version: Get the latest LNP version and migrate your save...
Capping volcanoes and pools:
It's perfectly doable (I've done it several times), and so is no joke. As you've noticed, obsidian formed in the center just results in the formation of obsidian that sink and get obliterated when they hit the SMR (Semi Molten Rock) at the bottom (The SMR eliminates everything non living hitting it when it's covered by magma).
Thus, you need to make sure your water obsidianize the edge tiles of the pool (where the newly formed obsidian can fuse with the existing obsidian sheath circling the magma tube). From there it can then grow by flowing outwards, but you'll typically find that evaporation will eventually keep up with the flow so you can't obsidianize a larger patch from the initial water outlet. At that point you close the drawbridge upstream (you've built one of those, right?), plug the original outlet (or just close the drawbridge if you've got one), extend the water tunnel and make a new outlet over a tile adjacent to the obsidian cover you already have so you can continue the process. You'll probably have to repeat the movement a few times to put the lid on completely.
There are two additional things you can do:
- Build walls to guide water. If you block the spread of water in some directions it will have to flow in the remaining ones, meaning you can get it to flow where you want it. Using that method you can actually cover the complete top from a single water outlet, but the time to build those walls is probably longer than to re-adjust the water outlet, and the builders have to work in a place where they risk attack from both magma and cavern creatures.
- Manual bucket obsidianization: If you get some pesky single tile(s) that just won't get any water onto them because it dries up, you can of course move the water outlet, but you can also build a stair up two level (i.e. an Up stair with a Down stair on top of it) beside one, or ideally multiple of these pesky holes and then designate the tiles in the air beside the stair as single tile pond zones (pPf or Ppf I think). That will cause dorfs to fill buckets (and make sure you have enough, or you'll get cancellation spam) and carry those buckets to the top of the stair to empty them at the designated tile. Once emptied, the falling water will obsidianize the tile beneath, plugging that hole, at which time you can remove that pond zone. Note that the water has to fall at least one Z level or it will just be converted into steam when poured on the magma (there might be some percentage chance that it won't be converted to steam, but if there is, it's low).