I might consider switching to permethrin. It is effective against most mites and a lot of other things including mosquitoes, safe (at least as much as frontline), affordable, and very easy to use. I bought it to keep mosquitoes off my goats and discovered so many other uses! A $10 trigger bottle labeled as horse fly spray has lasted me more than a year and a half with 2 large goats, 3 large dogs, and 60+ chickens. I DON'T just use it and assume my preventative measures are working. I use it when I see something (lice once), and have verified I wasn't missing something by taking the bird to the vet for inspection. He confirmed there was previous evidence of external parasites, but none currently (at that time). Like you say though, it is an ongoing process! You can spray it on your walls and in your boxes, and has a residual that keeps killing for a period of time depending on the species. It is topical only and LABELED for use in chickens, requiring NO withdrawal time for eating eggs. Without the consultation of a vet or some solid EVIDENCE (study references) that there is NO residual to worry about in the eggs, I would hesitate to use and might even go as far as to call it an illegal use!

I promise, I'm not trying to pick on you and only mean to be helpful. Like with anything, you have to do what makes sense to and work for YOU.
The vaseline may work for the large cracks you can identify, but I'm sure they can find smaller crack elsewhere. Bonfire was a good way to go. That sounds like a big nightmare. Glad you are past it now. It's a lot easier to keep up with things than try to fight infestations (I know they happen and aren't relative to your cleanliness). Oh boy if you have a lot of birds or no one to help it is sooo much more difficult and time consuming to treat them. Are you able to do so with out chasing them all around? Or is that another one of your challenges? Thank you for sharing your experience. It's very helpful.