A great aid in removing frozen fasteners is car jumper cables. On the plus side of the cable, clamp a stainless or bronze bolt. I’ve used 5/16” but probably most any size would work. Clamp the ground side as close to the offending fastener as possible. Put the plus side bolt against the head of the frozen fastener. You essentially set up an arc welder and generate a lot of heat, like cherry red kind, directly on the fastener. It’s worked every time I’ve tried it on my 40 year old mast except one. Heat the fastener and spray with penetrating oil. Often takes several applications of heat and oil so have patience. Getting SS fasteners out of my aluminum Self Steering vane took periodic heating over four days. Use TefGel or Lanolin when you put the fasteners back.

Heating with a torch has worked but is difficult to control. Worse, you end up destroying the fitting when there is heat sensitive material involved like plastic sheaves, etc. The Jumper Cable trick is the only one that’s worked with minimal collateral damage. It's also worked way better even when I've tried other heat sources.

A battery powered impact screw driver makes getting the fasteners out way easier especially those that have been softened up with the battery cable trick. Try the impact driver first, but not aggressively, before resorting to heating. These drivers will twist off/break the fastener at least up ¼” so be careful. I’ve got a 12 volt Hitachi but all the other tool manufacturers make them. Voltage has been growing on these but I’ve not felt the need for more than my 12 volt one. The biggest problem with them is the straight slot bits are relatively narrow and it’s easy to tear up the fastener slot if you are not careful. The flat bits also shatter pretty easily. I went through way more than a dozen R&Ring my 40 year old mast.

Penetrating oil seems to have limited effect on stainless/aluminum corrosion without the battery cable heating .

Heating with a torch can work but is difficult to control and spreads the heat over way more area than just around fastener. Worse, you end up destroying the fitting when there is heat sensitive material involved like plastic sheaves, etc. The Jumper Cable trick is the only one that’s worked with minimal collateral damage. Mapp Gas torches put out the most heat and seem to be safe with Aluminum. Acetylene can leave you with a puddle of aluminum.