Def lay on cork. It will help reduce noise through board and enable you to create an edge to ballast against.
Re the track- use the pin vice to make holes for track pins although if your going to glue and ballast the track at least in the scenic sections then pinning won't be necessary, you'll just weight the track down while the glue dries. Ballasting will be done after that providing further stability.
If you haven't seen already I strongly recommend you look at Charlie Bishops YouTube site Chadwick Model Railway. Apart from Charlies wonderful presenting skills you'll find a wealth of helpful tutorials on building a layout- I wish it had been available when I started my layout! You'll find he has a link to various suppliers, one of which is a firm in Aylesbury supplying large rolls of cork sheet at good pricing.
I've read through your thread today, and although both era and scale is different to my interest, I've found it a thoroughly interesting read and look forward to following the build.