Would hanging upside down for 15min 2x a week prevent brain blood clogs and strokes?

No, but it was linked, at the height of the "inversion" craze, to occasional visual injuries and a few other problems. Leave the hanging upside down to bats, sloths, and such critters.

I hang upside down on my inversion table 3-4 times a week. I find that 5-7 minutes is all I can do. I do it to relieve tension in my back and knee joints. I has really helped me. Not sure of the impact on prevention of strokes.

No

I don't think so...