Yes you can adjust Z a large amount, it is a 421 mixer so Z moves 4 times as much as B, 3cm B adjustment is similar to a 12cm Z. There is quite light forces on Z so a pigtail could be made from doubled 100-200kg line with little issue. The majority of the kite forces sit on A and B, less on C then even less on Z. It's why Z shrinks the most because the lines have the least load. On flysurfer about the only bridles that hold a consistent length is A, but they still shrink a little, especially on the tips where the density of the bridles or spacing is much tighter so again the bridles have less load and in turn shrink more.
The kite being harder to relaunch can be for a few reasons. Off my head here are some.
1 the camber has decreased which can make relaunch more difficult. Not much to do on that except increase it back. Something like a WAC line improves relaunch, with other positives and some negatives.
2 you could be running out of travel on your pulley lines because of the pigtail blocking the pulley. Simply move it around so the knot doesn't block the pulley.
3 you have changed the range on the mixer. Which is likely after I checked how the ozone checks/corrects the mixer. Basically the mixer check with Ozone relies on the A main length to compare against, so basically if that shrinks then so does the length you correct the pulley line and it in turn is then too short. End result is less depower and power, less movement of the canopy, so relaunch will be harder especially on the bigger kites which need more mixer range because of their size. To check for this is easy just measure your total A main so PA1+PA2( I am hating the naming even more after looking those up
). To correct it you'll need to either extend Amain to it's correct length then do the Ozone way of checking/adjusting or adopt some of the flysurfer method and adjust the mains for B and C in addition to adjusting the pulley line back to it's correct length. I also add it is not quite as simple as just the A main being the correct length, B and C mains need to be the correct length too :-/. You'll need to set everything A B C mains and the pulley lines to be there correct length. Now I am starting to understand why flysurfer adopted their method.
4 you moved the mixer to depower more, but I can't think of that happening without the mixer range increasing so :-/
5 and this is very likely, by extending Z you have moved the power point further away so your relaunch strap is less effective. I would just adjust your line lengths, shorter rears or longer fronts.