I honestly don't know what the current valuation for a 558 would be...i'm sorry. They sure are pretty rifles but i've no experience with them, only saw pictures when i was looking for my Tikka rifle but it was a long time ago.
Perhaps you're correct as to the shared platforms...the reason i think that might be true is when one searches the GRS site for fitment it links the 595, the 558 and the 590 together.
With respect to the 595.... GRS offers 4 different stocks, i strongly considered purchasing one of the GRS laminate stocks like the sporter or the hybrid as i didn't want to bugger up my original stock but figured life is short might as well enjoy what i have. I believe Eurooptic carries the GRS line. LRI mentioned in a recent post that they have experience custom insetting GRS stocks to barreled actions.
Here's the link to the Eurooptic stocks that should fit:
https://www.eurooptic.com/search.aspx?keyword=tikka+595
I've communicated with GRS in the past and they're very responsive by email (albeit it took a couple of days....which makes sense as they're located in Norway).
It does look like Midwest Gun Works has some (but not all )spare parts still available like the firing pin, bolt body, recoil lug etc.
sorry i couldn't be of more help