I have worked around this a lot, and have a few notes which might help out!
First on the "1m" option, yeah if it's sliding you need to deselect and reselect into your group, then I also usually move the slider away and back to 1m, before AND after starting the move for the pieces I'm moving to be extra safe. But pay attention that there are TWO "1m" options, and the lower one is actually 0.5m. A helpful feature, but confusingly labelled
For getting your roof pieces to line up, the following fiddling SHOULD fix it up:
Click into your build group, and select one of the roof pieces that is at the height you want the rest to be at. In the build menu (bottom right) there should be a "Select Grid" button, click on that, then click again on the correct roof piece. You should hopefully find now that moving the slightly offset pieces puts them onto the vertical alignment with the correct ones - make sure you have a non-zero vertical movement selected at all times when adjusting them so they stay that way. If you are lucky, you should be able to select all the bad pieces and move them in one go, but I often do it piece by piece or delete and replace them all to ensure the alignment remains correct.
If at any point things get out of line again, re-do the select grid thing to a piece that is correct.
The select grid button when working within a group has saved me a lot of times when it turns out things are being placed slightly out of line, this happens not just vertically but also sometimes on the horizontal grid for me, although only by small increments so it sometimes doesn't stand out until you've placed dozens of things slightly misaligned with the rest of your building!
Hope it fixes your thing too!