I finished a MTM build a few weeks ago. You should be very happy with the outcome.
I don't have history with that particular speaker so I can't speak to the accuracy of the xover. However, one thing to note is please obscure the component values (caps in this case) when posting as those are not in the public domain. Most people either cover them up with tape or use a photo editing app to scribble them out.
I just covered my xover with polyfill (acousta stuf in my case), but mounted it to the stiff layer of no rez by peeling off the foam to create a space for the board.
I also put some fill (not a lot) behind the woofers. I would imagine it would be similar to the X-LS in your case. I just put some in there, fluffed it up and let friction do the rest to hold it in. The braces in the MTM provide a decent "floor" for the fill, so I'm not too worried about it. If there was less bracing I probably would have use a little spray-on glue on the sides. I'm sure others have better ideas.
I didn't seal the inside of mine, but learned later that it is indeed a good idea as a precaution. Whomp, whomp
(is whomp, whomp a thing in GB?). Luckily, I'm in a temperate, low humidity climate so it shouldn't be a big deal. A few quick coats of wipe-on poly, shellac, or lacquer for example. should do the trick.
3/8" roundover is fine both inside the woofer holes and on the front baffle. I went with 1/2" for aesthetic reasons. Some folks don't roundover the top, probably to keep the veneering easier, but if you are doing duratex then you don't have to worry about that part. If you are at all considering veneering, you could leave the top edge square and go back later to round it off if you end up painting them. Veneer is a great way to finish them, but it can be a little pricy and time consuming.
Good luck and I'm looking forward to seeing your progress.