Reverse the hard drives even if not making room for a pump, less cables means more airflow and everything cooler. It's just always good practice. Also looks cleaner and makes the system easier to work on.
Loop order doesn't matter. Reservoir before pump, which you're stuck with anyway

, and then just try to minimise tubing length. Ideally avoid using 90 degree fittings.
The pump will be fine, especially if theyre good radiators. High restriction loops is what it was made for, it'll run cooler if it's having to push harder too. If eventually you decide you want higher flow rate for some reason, put another ddc in series with it (without its own reservoir). Still don't need dual loops.
Fans matter very much with this, I believe we just found the problem. Particularly with high fin density radiators (like yours) , crap fans wont be able to push enough air through them. case fans are optimised for high airflow, which is no good to you here.
You need high static pressure, the expensive noctua and inexpensive yate loons are loved for this. I use scythe s-flex. It's possible, though I wouldn't bank on it, that three fans with shrouds would completely turn your radiator around.
The money saved on not getting another ddc will get you a godo triple radiator, and some yate loons. I think this is a better idea. A good triple, with 3 shrouds and 3 fans, really should cope with that heatload. Temps wont be lovely, but far from crashing. A good triple and a good double if it fits and youre laughing.
One loop over two is very much my opinion mind, I opened a thread about it ages ago. Have a read and make your own decision, a lot of good information in it.
Dual loops. Why?
I've suddenly become aware that I'm steering you down the path I think is best rather than that which the community seems to be in love with.