Re: Many questions, wanna make a purchase!
The vibrations should be minimized, when RPMs get minimized through a smaller sprocket.
(One of the reasons I don't want a 41T sprocket...)
I understand most men can't maintain 30MPH (never said he was able to), however, most men can ride 30MPH, even if it is for a short time.
For the bike, I don't think there's much of a difference between riding 30MPH for one minute, or for half an hour, as most of the bike is solid fixed, and very little screws can come loose.
As for the frame, beach cruisers have very solid frames. Some mountain bikes also come with big bars.
The fixie bike, or sports bikes may be too thin though...
The Chinese bikes of today,are about as good as the middle class bicycles of the 1980's made in Europe (or USA).
They have more comfort for lower prices, but generally the frame is a little less good.