Other than the race condition (which should just be fixed), I don't see the point in expiring the cache so frequently. Let the LRU do its job.

but in your version you're also updating the cache when updating a node. so it's the same work actually.
less efficient? I don't think so. Reads happen much more often than writes.
More complicated? I don't think so. If I update something i simply delete the cache entry. Then let the next read create the cache again and don't care. That's even less complicated because in the read version I create my data structure how I want it. When I have to create the cache entry when updating I have to do the same work, although at that point I'm not reading the whole thread.
Like I said, I'm caching the whole thread. When updating a node, the whole thread cache entry is deleted. The next read reads the nodes from db, puts them in my wanted data structure and then a) caches that and b) passes it to the template. If I had to create the whole thread data structure in the update before the post, it would seem more complicated to me.
update: forgot the "pass to template"