Clearance is distance as the crow flies - i.e. a straight line through air. It can go round corners if there is a solid insulating barrier of adequate breakdown voltage in the way, but is always the shortest possible path, including through a tiny crack between two insulators only mechanically fastened together. Its the distance an open air arc has to jump for it to fail.
Creepage is the distance measured along an insulating surface and, together with the insulating material and its surface condition and treatment, determines its resistance to failure by conductive tracking.
Clearance distance is the dominant one - if its no good, creepage distance is irrelevant.
Your design violates clearance (and creepage) at all the optos between pin 8 and the Line track under them. There's a dammed good reason why their package omits pin 7! It also fails between their output tracks, which may have up to 253V RMS between them with one channel on and an adjacent channel off. For that region, according to
www.creepage.com, and assuming pollution degree 2 you should be maintaining 1.5mm min. clearance and 4.1mm creepage for functional insulation between tracks/pads on the same side of the board. You'll get a bit more creepage rating from your confomal coating, but unless you are set up to test the board surfaces for contamination and surface resistance before coating, then perform a 100% UV inspection of the coating for defects in critical areas, don't count on the extra the coating gives you.
It also probably fails clearance between that Line track and the copper linking pins 1, 3 & 4 on the LV side of the optos. You need 4.0mm clearance (Reinforced insulation) if any metal part on the LV side can be touched or if there's external LV cabling. It fails creepage at the isthmus between the slots between the third and fourth optos down. You need 5.1mm.
N.B. the optos aren't rated for pollution degree 3 or higher - you are going to need that sealed case with cable glands and a filtered vent if there's any chance of it being used anywhere near a fog or smoke machine.
Other design fails are the lack of any provision for snubbers or gate pulldowns to redce false triggering if it proves problematic in service. That sort of thing needs to make it into the prototype and any beta test run even if its DNPed then once you have enough boards out in the wild, *IF* you've never needed it *AND* you need to do a board shrink, you can delete the extra footprints on the next respin.
FR4 has adequate insulation breakdown voltage for a Line track on one side crossing a Neutral on the other to be no particular problem unless the board is compromised by excess heat or mechanical stress. I wouldn't trust Line/Neutral on one side over a LV circuit on the other. A thicker PCB will be stronger, but electrically its not essential.
Minimise your slot lengths - there's no point in any of them extending more than is required to maintain >7mm creepage as that's all the opto bodies give you. Maintaining mechanical strength is essential, and if the board cracks at or near the end of the slots, the cracks can trap contamination and encourage tracking over or through the FR4, or a power track can crack, arc and char the board till it fails..