I do believe it's possible for a user to copy the jutsu off a newbie like genma and that lot but when someone like Minato uses it without hand seals there's nothing for the Sharingan to copy which means no FTG.
I believe once a technique is mastered to a certain level/ degree you no longer need hand seals to perform said technique example is Naruto and his mastery of Rasengan.
you know ftg is an s-rank technique right? furthermore, the seal is already in place and no uchiha has shown a legitimate sealing technique. so the sharingan will copy a hand sign which will do fk all