Your analysis was excellent. Thank you. If I do figure out an option and do get a setup, any chance I can tip you to help me get it?
If it's anything I can help with, I'll gladly do it free of charge.
note the antminer L3+ uses 120 volts it is okay
the s15 use 240 volts it may be better.
To clarify the subject further, S15 will run just fine with anything between 200 and 240 VAC, it won't run on 110v even with 1 hash board inside.
Or why not a S19 with only 1 hashboard connected? It doesn't cost a lot of money to buy a broken S19 online, and could give a good ratio BTC/kwh.
Not a bad idea if the price is right, I'd just make sure the other 2 dead boards are inside the miner, or else the miner will run pretty loud as air will "prefer to" travel through the least resistant area which will be 66% of the miner, so the remaining little air will need to increase to cool the working board and thus increase the fans and the power consumption, if the dead boards are not there, then OP needs to do some modifications to the miner.
One thing to keep in mind, S15 has only two fans on the intake, none on the exhaust side, the S19 has 2 on each side, so the S15 is a lot quieter to run, especially during the boot/fan speed test cycle.