If you can make one kata as fast as it took A to cross that distance, then you would still need hiraishin in order to keep from getting punched, as A would have been able to notice any minor change in movement and could have adjusted himself to it.
Tobirama using it while holding both hands are on the sword is questionable evidence. All we are shown is him slashing and using the technique; we have no idea where the marker or chakra of his was located at in order for him to perform the flying thunder god technique in the first place.
While this next part is more theory than hard manga evidence, he could have formed the kata even still while holding the sword, using one hand to hold the sword and the other to make the kata. Then, when he warped, the other hand is put onto the sword and the user slashes in a single fluid movement that would be very much possible for a kenjutsu expert.
And the speed of making a one handed kata, and then moving it to another location of choice can be potentially quite high, especially in the case of Minato. While that is the best explanation that I can give for him holding Kushina with both hands, I still persist. Why suddenly does is he shown using handsigns whenever he uses the technique as of the recent two chapters in particular?
I find no evidence granted that convinces me otherwise that hiraishin does not, if not since introduction but since these latest chapters, require a handsign to perform.
Several examples of him using a hand seal in the latest two chapters:
http://www.mangapanda.com/naruto/639/5http://www.mangapanda.com/naruto/639/15http://www.mangapanda.com/naruto/639/16http://www.mangapanda.com/naruto/639/17http://www.mangapanda.com/naruto/640/3http://www.mangapanda.com/naruto/640/4 [Him re-appearing after the previous example]
http://www.mangapanda.com/naruto/640/16 [This one is a bit ambigious imo, since it could go either way depending on how you look at it]
I think got all of the instances in which he used Hiraishin in the latest chapters, except for that second to last one where only his head was shown before it cut away to him charging and obito's internal struggle.
*edit: The bottom line is, at least in the majority of them, is very clearly seen using a handsign to activate the technique. If he had the option to use it without a handsign, then why wouldn't he use it?