Pins help and the hollaender setscrews bite pretty good into the aluminum. Other setscrews not so much.
I'd only run the 6061-t6 temper for pipe as the t-1 will deform from the setscrews requiring you to retighten frequently and ovalizing the tube. If you pin i'd seriously consider reinforcing the inside of the pipe with some 6061-t6 tubing whose OD matches the ID of the pipe
I made a couple of 10' paddle cat frames using the 3/4" fittings and they have held up well. Of course this doesnt support much weight or the stress of an oar tower
Hey, if you want me to take a dump in a box and mark it "guaranteed", I will. I got spare time.