This trigger actually only fits 1911's.
Yes. The P320 Carry M18 uses full sized magazines so the 9mm 17rd magazine is correct for it.
The Sig Sauer Caliber X-Change kits do include the guide rod and captured recoil spring.
This model does not have a manual safety.
The FX910 ships with one 30rd magazine and it will accept Glock compatible 9mm mags.
This slide is stainless steel.
It will not unless you change out the controls. The slide stop / release lever and take down lever are different on the SAS.
This kit is compatible with the P365X as it uses an XL grip module.
This is a compact slide so it is for a 3.9 inch barrel. A subcompact slide will accept a 3.6 inch barrel.
The painted portion of the recoil spring would be opposite the front of the barrel (at the rear).