Hey Chris
Been there with the fire code deal! You can notch the studs in the area of the flange on the opposite wall of the firewall to get that additional 1/8-1/4″ needed. I would recommend keeping the 5/8″ drywall off of the firewall till tub is set and then slide it down behind tub flange when everything is plumbed and done
And you have the right idea on setting the tub with no access. Its a pain to line things up and constantly be lifting the tub up to adjust things. Just try to account for the 1/8″ or so that each piece of pipe will sink further into fittings when actually glued
And if you plan to set with mortar, make sure you mix it pretty loose (easily pourable) so that you are not fighting the mortar when setting tub
thanks
steve
