Yeah, since daifunkas autocombo is dropped because of the steel rain, it is a reset, and they block again.
The first two times i show it in the video i have 12 nails, the last time when Ragna has 75% hp and dies from it, that's with 6 nails. Anywhere inbetween that it works. 6a super works yeah. Really after the guard crush from the burst, you can go right into daifunka if you want to. 6d > steel rain may be slightly better looking back on it, but only for the mentioned damage. If you use this setup, chances are you won't be getting another 50% heat to use FRKZ.
I tested this for a few minutes, 5a>5b is all I could seem to get to link off of it, afterward they can tech before a j.a etc.
Yes, you can do it without offensive bursting. The second time I did it in the video, the offensive burst was actually aimed at the barrier, which is why Ragna when into DANGER mode, and then the 5c was what actually started the nails hitting.