1 . I don’t have the facilities to test a c.t. down to 30 Hz. I think it’s reasonable to assume that the YHDC devices that we normally use would exhibit a roll-off at that sort of frequency, I cannot speak for other manufacturers/types, but I suspect some will be better than others (especially those containing more iron!). I think a trawl through manufacturers’ data sheets will be in order - maybe some emails as well. In the past Magnelab (US, and quite expensive) have been very helpful.
2 .
Yes it does
I don’t think that comes from an official source (link please if you can) and I certainly wouldn’t recommend it, it will introduce a significant phase shift if nothing else.
3 .
I think that will very much depend of the bandwidth of the c.t.
I agree with your concerns. It is possible to obtain an isolated d.c. measurement, but tends to be cost-prohibitive for most people. As for measuring the current, that should be relatively straightforward. I’ve read your note to Dan (I’ve been advising him too) and I suspect the way to address those concerns is to have a mounting plate made from thick Paxolin or equivalent, with a couple of holes drilled, through which you have a couple of brass bolts with nuts & washers. You then need some copper braid (as used for car battery earth straps) and the requisite number of crimp or solder lugs, so that you can terminate the fixed cables to a bolt, then use the braid from there to the ACS sensor.
There’s no difference, when you think what the rms value means! 
The CSA you need will depend on the installation conditions and the protection it has (like every other cable). Protected by a m.c.b. or a current-limited source and in conduit, it might be 10 mm², also in a thermally insulating wall it could be 16 mm².