James,
First, I am hoping someone more experienced chimes in. But against the odds I have been very successful with logs (normally I screw up projects to learn how to do them, this was a rare WOW!)
My take is that any tree that is partly dead is likely to be already colonized into the area that still looks alive. It takes very little cambium to keep some leaves, but there may be extensive decay already. This is very likely to contaminate your logs. Fresh windfall tree is fine, but beware anything that has a large dead area. You mentioned the spalting, which is macroscopic colonization. It can be much more extensive, especially longitudinally (less circumferentially). Anything lying on the ground I would presume contaminated.
Second, I would inoculate when the log is not going to freeze completely for at least a few weeks. Surface freezes are fine. The fungi survive freezing once established in nature but the spawn is more sensitive which is why it isn't shipped frozen. It would probably work but you want the spawn to vigorously outcompete other fungi, which means it needs to get a foothold. Lower active spawn count might be a problem. I think you could do pegs any time in my climate, but again, you will introduce contaminants and if the whole depth of the peg freezes it would be more risky. I think the main issue is not so much the temperature but the shifting humidity during freeze/thaw cycles as
water sublimates out of the wood. It's really hard to keep the logs moist with a lot of freeze and thaw, partly because my hoses are turned off and because I am not out there as much remembering to do it.
It is so much work to inoculate, I am really OCD about it. The only logs I inoculate are fresh cut, off the ground as much as possible (I often fell the tree and limb from the skinny side, taking the logs without them ever touching the ground), and then stacked on the cement patio and inoculated 1-2 weeks later. I have heard this allows some degradation of the antifungal compounds but I really don't know. I would err on the side of early rather than late inoculation. I have been sealing the ends, but I did an unintentional test by forgetting to seal two of them and they did fine. Probably won't bother anymore. Then I stack the logs on the patio in the shade and water them every day for the first week if it is dry weather, then maybe 3 times a week the next couple weeks. By then I am distracted by some other
project and forget to wet them unless it is really dry and I am watering other stuff but it is generally pretty moist here. After a couple months I half bury them in dirt and put some wood chips around them. So far batting 1.000. I also really put a lot of spawn in the logs, normally 6-8" longitudinal and 4" circumferential spacing.
That being said this is only oyster and shiitake so far. Humidity is the key I suspect. Sealing is really just a way of maintaining internal moisture, I don't think it really prevents contaminants with aggressive strains. With more difficult strains it really might be important.