None of our rifles were ever varnished, ever......... Just try holding a varnished rifle in the rain - of which Malaya in general is well known for. A goodly dose of linseed might help with the warped fore-end but it will not cure it. Dunk the whole thing in very warm to touch oil and let it soak for several hours. Don't think of varnish keeping the weather OUT, think of it as a medium for keeping the wet IN which isn't quite what you want!
When there was any sign of rust or corrosion, and there was inevitably some, believe me...., we would drift out the fore-end cap rivet (I seem to remember that they were brass on the No5... correct me if I'm wrong after all these years) and make good the end of the wood. Then, importantly, glue and plug the rivet hole, make good and then round the end to suit. Then dip fore-end in linseed for several hours. Done hundreds of themPolice, para military Police and Army, from right across the Far East, they all came through the same big UKworkshops