We recommend this reflector as it comes with 4 different grids. https://www.bhphotovideo.com/c/product/1577807-REG/westcott_4722_55_degree_wide_umbrella_reflector.html

The diffuser may interfere with many square LED panels.

Yup. It's amazing!

I have used the diffuser for my 7' umbrella. I can't show a sample, but I (as an amateur) was very pleased with the results. A professional shooter friend was very complimentary on the visual effect on my model's skin, very soft from the size and beautifully diffuse from the sock.

No experience with the Neewer but the fit on the Westcott is fairly generous so it should work. Best if you could get exact diameter of both brands to compare.

Yes. It will work with any speedlight support of 3 speedlights. I ussually use it with the one that mounts directly on the shaft of the umbrella and requires another to support the umbrella on the stand. Also have a conventional 3 speedlights mount, but prefer the other because distributes the speedlights around.

I only use it for my Elinchrom BRX 500 and yes, it's big enough for most light. Regards,Pat

normally the diffuser fabric has a sock-like opening in the middle from which you insert only the bulb section of your flash unit - leaving the back end of the unit outside. If, for any reason you want to keep the entire module outside you could do it if you have a sturdy enough umbrella mount, but to what effect, I cannot comprehend.