I find the scrim works if you place the first couple of inches gently into the paint with your fingertips, then smooth that first bit with a filling knife. Now it won’t slide, so you can unwind it gently off the roll and treat it like it is going to crease at any second.
If the crack isn’t dead straight, you should be able to develop a gradual bend without too much trouble. The second it creases though, with too much of a bend! pull it back to the start of the crease, trim with scissors, and start with a new piece repeat - bed first couple of inches in wet paint, flatten with filling knife and carry on.
The idea is to use a coat of Bullseye to provide a superior base for topcoats of emulsion. I seem to recall you have already painted out the walls in Bullseye, but now with the retro repairs to the crazed paint etc, you have a judgement call
Either spot in the patch filling with bullseye, which will leave you with a nice overall “bullseye surface” to apply finish coats too, or if there is a lot of filing to touch in, give the whole wall a light sand and paint the whole wall with Bullseye. Then 2 x emulsion, denibbing between coats.
If you apply acrylic emulsion on normal bare plaster, or with areas filled with Toupret fillers, you wont have flashing to worry about, so dont defaut to Bullseye as a spot primer. At worst for belt and braces, have a mini roller with a sleeve the same texture as your main roller and spot roll the emulsion over any fine surface filler ahead of painting the wall. Literally spot roll a wall then drop back and start the first coat straightaway. But really, as an idea of coverage, 2 coats of an acrylic emulsion in light and medium bases have enough opacity to take bare plaster to a finish.