Saratobi knows FTG
Tsunade knows FTG
because they are hokages and they have access to secret jutsus as well as how to perform them. They just can't use it. It's more special than minato death reaper jutsu seal. Sarotobi third hokage was able to use death demon reaper seal but not at the same level as minato's. I am pretty sure he attempted in using FTG as well but failed. I wouldn't be surprised he wasn't the only one who attempted FTG. There is got to be a special contract or ritual before being able to use FTG. A restriction that only indicated shinobis can use it
tsunade does know FTG but she can't use it. In order to be able to use FTG there has to be some kind of contract and special ritual that will link you with the markers ( signs)
Minato is able to know where each marking is and he is able to choose which mark to go to. For example, among the many other markings, he knew the moment kakashi withdrew his kunai. Minato knew it was kakashi withdrawing his kunai and teleported to his location in kakashi gaiden. It's just not a simple teleportation. He also has connection to the kunais. He can tell the location of each of them and decide whether he wants to go to one or the other.
