I am in the process of adding a pool heater. 240V (60A breaker/40A max load/3 wire)

I also want to put this in a GFI. I have a SquareD/homeline box, which does not offer 60A GFCI breakers. So, I purchased a spa disconnect to place outside (in addition to the standard 60A breaker in the panel)

However, the run out there is pretty long - about 215'. So, I don't want to pull an extra neutral wire just for the hell of it. (I'm running 4ga wire to the heater due to the length of the run). From what I understand, 6GA wire would have been sufficient for the 40A max load and that length)

The GFCI that I got looks like it requires a connection to neutral. So, I can do things one of two ways.

1. Spa disconnect in basement at panel, and wire up the heater direct. I'd rather not do this - I'd rather have the disconnect outside if possible. However, I know that I can fall back on this. This would (at least) shorten the length of the wire needed between the panel and the GFCI.

2. Spa disconnect outside. I do have some spare neutrals out there already (for accessory power, pump power, and chicken coop power... 2x20A AFCI/GFCI breakers running over 8ga wire..

Question A Can I tie the neutral on the spa panel GFI to the neutral on one of the 20A return legs? Would this cause issue with the GFCI/AFCI function of on the main panel? I assume that the neutral wire is ONLY used as a reference - so I shouldn't be adding any real current to the return

Question B Can I tie the neutral on the spa panel GFI to the return ground? Technically, both are tied to the same bus bar in the main panel. (my question remains the same with the neutral in the GFI outside being more of a reference, and not really responsible for any "real" current needs)