Whoops, sorry guys, got distracted...
OK, so Faber-Castell's Tack-It is this really soft, stretchy re-usable putty:
https://www.amazon.com/Faber-Castell-Multipurpose-Adhesive-Non-Toxic-Removable/dp/B01FP5QXYQ
that IS sort of green-ish blue, but IMO that is useful so you can see where you put it. It is kind of sticky but in a good way. Discovered it when a little square of it came with a Kumik doll wig (which I guess can be switched if u want to give a doll a different wig.) (Apparently there is a white version on Amazon as well, but one green pack has lasted me a couple of years).
This stuff STICKS. It doesn't dry up, tho I don't re-use pieces. Ordered some from Amazon and use it for all kinds of things - like the only reason the SSC Jawas
are standing up on the Hasbro base in my Jawa pic (SW forum) is wads of Tack-It. It's a great cheat for posing.
So for Cap, pinch off enough to make sort of a 1/4" or so sausage, wrap/mould around arm peg at the base/sides of peg (like u are wrapping peg with chewing gum or a rubber band, attach arm. Leave the END of peg uncovered (in theory to make it easier for the *&^ magnet to do some work). Don't wrap peg so thick u can't get arm on, just give the sides of the peg a good coating of Tack-It like u are spreading peanut butter.
U can use as much as you feel comfortable. IMO more is better; u can always pinch off any squeeze-out (but don't OVER pack it/force, just get a secure hold by feel. U may have TINY wiggle, u can tell when u have to work a bit to get arm off). It's sort of like making a custom sticky rubber gasket (apparently this stuff is made from real rubber).
The point is to get a secure hold, not to stuff so much Tack-It in it "strains" the poly or u over-force the arm. If any bit of the stuff gets left behind, u can pick up with another wad of Tack-It, forceps, toothpick, etc. U can remove arm and add more or less until u feel happy with hold.
I wouldn't overheat this putty or super-freeze, but I'd never do that to my collection anyway. I took one look at Cap's arm and reached for the Tack-It. Because, I'd seen too many pics of Sideshow's Kier that had an arm pegged the same way, where it slipped and shattered.
Shocking IS didn't use better magnets hadn't seen that before in an IS piece.
Hope that helps. If I really wanted to be sure, I'd glue with E-6000 (which can be peeled off from hard surfaces with some work), but for me Tack-It is kind've a go-to
. (E-6000 tho u had better be sure; it dries to a very tough, rubbery epoxy.)
Don't know if anything else is comparable for "reusable" adhesives. Only problem is it's kind've seagreen, so if u use for posing u have to watch it's not peeking out from anywhere. Over several years have had no problem with drying, oiling or staining. It's sort of like chewed gum so I would be cautious around using on cloth, tho have done that too (use tiny amount). Some of these re-usable putties seem to get greasy/oily over time, but never had a problem with Tack-It. German engineering
. I've got Cap posed with helmet and portrait - I'd be REALLY upset if there was arm damage, but really didn't feel like gluing either. Can't imagine what IS was thinking. 
They seem to usually have pretty good magnets.
