I think the same is also true of Stainless Steel bolts! I did raise this on a Stag Forum a while back. I do think you are right that the "least worst option" is zinc plated steel. Stainless Steel or Nickle Plated bolts, studs or what ever are potentially (pun intended
) more reactive/corrosive in my opinion. No one challenged my posting, but I would also be interested to hear what anyone with a chemistry/physics/metallurgy background thinks!
As to the advantages of Nickle Plate ......... not sure but they are shiny !!
For my money I use standard zinc plated studs and bolts. In the head I open the holes out slightly to ensure they are clean and have a smidge of clearance. I give the shanks a good smear of copperslip, I am aware that some differ on this and use a ceramic grease but it has always worked for me. I have an old Land Rover with an aluminium timing cover with several long thin bolts holing it. 20 odd years ago I removed it and when I replaced it Icleaned out the mounting holes and coated the zinc plated bolts with copper slip. I recently had the cover off again and had no trouble removing any of the bolts. Not quite the same as a Stag or Slant 4 cylinder head but similar - long steel bolts through an aluminium casting.