So what is the correct play rules for the double-Jack strategy that is so strong against other BM strategies?
First, my usual rant:
Double Jack is not "so strong against other BM strategies". It is almost
never the best BM strategy on the board. Sure double Jack beats Smithy+money, but Jack+Smithy+money beats double Jack. Similarly with Envoy. And Courtyard+money beats double Jack straight-up. I posted in another thread that double Jack also loses to a Jack opening into practically any decent $5 card (bazaar, cartographer, council room, duke, embassy, explorer, festival, inn, lab, library, mechant ship, mountebank, rabble, royal seal, stables, stash, torturer, treasury, vault, venture, wharf, witch). Jack is a great
opening for BM strategies, but it's just an opening...
/double-jack-is-overrated-rant
So play strategy for Jack:
Early on, a discarded estate will likely be replaced by copper, so the question is if you're at a critical price point. If you have $5 in hand, you probably want to discard it to hope for a copper to get you to $6 and a gold. If you have only $4 in hand, you may want to keep the estate to trash, unless there is a $5 card such that you'd rather have the 5 and an estate rather than a 4/silver and no estate.
If it's a copper on top, it's probably not worth potentially missing a gold/province just to cycle the one copper, so I keep it.
You basically never want to trash an action card (like a second Jack) with Jack. If you had a good reason to buy it, you probably want to keep it.