First off, double Jack is super overrated:
http://forum.dominionstrategy.com/index.php?topic=2634On this particular board, I would probably open with a Jack to trash Estates, then go mass Monument. Adding VP chips is going to be better than adding a bunch of Silvers, which is all a second Jack really does for you.
But dealing with your specific strategies:
You don't have any cycling, so Jack should be a better opening than Mine. Also, with Jacks trashing Estates and adding Silvers, it's unlikely that Crossroads will do anything positive for you. It ended up actually helping, but probably just Jack/nothing is going to be better overall.
Royal Seal is of course good in this situation. You say:
its just another silver that slows down your ability to cycle through the deck playing a bunch of Jacks, is it worth it?
but this kind of misses the whole point of a double Jack strategy. You
want to have a bunch of Silvers. If he skips Royal Seal and plays another Jack, the Jack also only adds another Silver...