Hi! First of all, I thank you very much for your post. I know my work doesn't look very bad, but comments like yours really make me keep going.
I'm not really sure about the meaning of all the "techincal" names you refer (I'm portuguese), but I'll try an answer.
As you saw on the photographs, part of the cruiser (back bottom tubes and seatpost) came from an old mountain bike, so those things you refer were not built by me, only the rest. I did not conduct any tests on the material, and I have no idea the weight limit, nor of it's flex possibilities (this sounds dangerous....)..
Finnaly, when I said "begginers mistake", I was not refering to the cruiser, but to the fact that I din not take any photos of the construction process. I really regret that, because the tandem was really in bad shape and it would be great to show you the evolution...
Do you know of any site where I can get frame measures for cruisers?
Thanks again