If I only need 1 or 2 rare from a treasure and plan to stop after that. I will pull 4 times x10 and start to pull single til I get a rare. That way if I lucky I may get a rare before the 70 mark and save some seals instead of waste it.
Example : If your rare come out as 55th pull but you only use x10, that mean you just wasted 5 seals in that treasure.
I've test on many accounts with single pull, they're all give demi rare after 10-15 pulls.

No difference at all, actually you get less 'demi rares' as you call them if you single draw but not by much, rares still come about every 60~.

If I decide to roll 50 seals into Kage, then roll another 10-20 in Sage. Will that give me a sage-rare, or are there individual counters for the different seal treasures?

different treasures count, get 60 in kage and get kage rare

individual counter for each treasure type