Your LAX layover won't be as long as you think.
If you are not a US citizen/GE you can expect a lengthy CBP plus you'll have to recheck bags, get over to the UA terminal from TBIT, and then reclear security.
By the time you do all that it may be more like 30-45 minutes and you'll have UC access due to samd day J travel.
I'm also firmly in the camp that the only way I'd fly Y over J here is if it were actually a direct flight. If you'll have to stop once for Y you should stop twice for J.