What should I do to prevent the balsa skin from crushing into the servo wells, wire channels and distorting the wing tube channel during vacuum bagging if the wing cores come with these already cut out? Tks

Can you fill them in with scrap foam? (but then how do you keep the scrap from getting glued into the hole?!)

I suppose I can. The filler foam will be glued to the wing skin , so it will come off when its time to cut the servo well . However this will not work for the wire channel !

I wonder whether I can get way with placing a thin sheet of aluminium or steel on top of the balsa sheeting over the servo well , wire channel and other cut out areas, making sure that there is generous overlap beyond the cut out areas ?

I have not had any problem with crushing using 1/16" Balsa skins with the vacuum pressure regulated down to 10" Hg. My rig has a pressure control circuit but you can regulate the bag with a needle valve and vent, you do need a vac. guage though. A pressure even lower than this seems O.K. down to about 6" Hg. This relates to 3 lbs/sq.in. which is an awful heavy weight on a typical wing. A good tip if using balsa, or anything that is a bit porous is to seal the inner surface first. I use a thin shellac solution which adds nothing to the weight and it dries super fast as it is alcohol based. I know guys who have't done this a nd resulted in a dry joint with the skin coming adrift with a bit of "G". the resin is sucked into the Balsa under Vacuum, leaving nothing for the joint.

You have a good point about sealing the inside of the balsa with shellac first. My home built rig also has a vacuum gauge and a vacuum switch made from car vacuum advance diaphragm . The switch keeps the vacuum within 1 psi from the set point. Only way to find out about crushing balsa over a servo well is to try a test piece .

Reduce the vacuum setting. You are applying too much presure