I'm getting more and more into the history of some of these Russian rifles...
Does anyone have any information on when Russia ran some of it's re-arsenal/refurb programs? 1970s? 1980s?
If you're getting into history... SKS, SVT, M91/30 were not Russian rifles (unlike Berdan, Berdan II, MN 91), they were Soviet and that's important to understand history of those rifles. Especially refurbishing that happened in various countries, back then all parts of USSR.
You question is too broad, maintenance, refurbishing and re-arsenaling was continuous process that had been happening until the end of USSR and many maintenance practices remained unchanged in independent countries like Ukraine, Belurus etc, after USSR ceased to exist. For the sake of simplification you can say "refurbishing" as we usually refer to it happened around 1950-1960x, for MNs and SVTs, not sure about SKSs.
I am aware of the Diamond shaped marks and such but is there any way to tell the year that your SKS, SVT or M91/30 was refurbed?
Diamond shaped mark IS NOT sign of refurbishing. To tell the truth there's no such thing as "refurbishing markings" at all. There are repair markings and there are arsenal markings, presence of any of them could mean many things, not just refurbishing. You never can't tell if rifle is refurbished just by single markings.
Generally speaking there's no way to tell when firearm was refurbished, however certain arsenal markings (MO, for example) includes the year. Can you call aresnal re-haul made in 1947 "refurbishing"? Up to you. But it was not what most people call refurbishing. Also certain markings existed only within certain time periods so that might give you a clue.
I am guessing that the rifles must have already been ten or twenty years old or so when they were refurbed?
As there were many programs to maintain firearm expect that they had various criteria, not just age of firearm.
Also, if the SKS was never a front line rifle and didn't really see combat (1949 - 1956 or so) then what was the cause for the re-arsenal program?
Who said SKS was never a front line rifle? Tell this guy to read USSR government decree #2611-1033, dated 18 June 1949.
As I said before - there were many programs and many criteria to refurbish rifle. Many of SKSs had seen very extensive service so they were refurbished also because of that.
Feel free to read this page and browse the site other pages for more information, there's BTW detailed info on grading. SVT-40 section for this site was written by me, but the whole site is maintained by one the the best experts is Soviet firearm - Ratnik.
https://www.m9130.info/repair-depot-modifications