How stable is the power? Freeze thaw cycles are usually more damaging. You would be better off just doing submerged agar wedges those are generally stable for a couple of years if you keep them in a cool dark cupboard 

So often just pairwise on plates but I have tried some pf-tek but without some downstream sequencing there will be no way to confirm what kind of crosses those are.

I have been working with fairly small block batches so I actually boil water before adding it to the bag. This dramatically reduces run time for my small inefficient steamer. I usually wait for the inner core of blocks to get to 99C and leave for 1 hour, then I switch everything off and leave it to continue pasteurising overnight. When I open up the steamer in the morning the blocks are still usually at ~75-85C.

Probes till holding up been through a few cycles but I have started placing them in pp baggies to keep the corrosion down.

Bit like hydrofoggers right you have to keep cleaning them. Any anti-microbial unless it is prokaryote specific will probably negatively affect your mushrooms. Even if it is a specific bactericide might I would prefer not to use it if possible.