no your right scrim isnt to deal with gaps as new regs say that there should be as minimal gap as possible (where as it used to be that you would leave a very small gap!) and yes one of scrim tapes purposes is to stop shrinkage but as far as im concerned you still shouldnt need to tape wall to...
you can get stop beads from the same place you would get angle beads, any builders merchants such as bradfords, it is just a bead you can use to put a stop in your plaster without reurning it around a wall or reveal
another way would be to fix some kind of batton to the edge of the wall wich pretrudes 3/4 mm from the wall and then skim to it and remove afterwards but as roughcaster mentioned it will be very easy to damage as there is no bead to strengthen it! you could always use a stop bead that way you...
ok im a profesional plasterer and personally i have never scrimmed wall to ceiling joints either! but having said that if the plasterboarding is done well there shouldnt be hardly any gap to scrim! so this all depends on how well the plasterboarding is done! as to wether you have the ceiling...
scrim tape is only used to cover the joints on square edged plasterboard when it is intended to be skimmed! joint tape is used on tapered edge plasterboard when it is intended to be dry lined! if you have already used tapered edge board then you should really dry line it as that is what it is...