I have certain plausibility issues.
I don't believe that a 5 year-old Ranma in the 17th century would be able
to read. Heck, I doubt Genma would be able to write and I'm certain that teaching
Ranma to read and write wouldn't be the top of his list of educational priorities.
I think having Ranma swim across the sea of Japan at the age of 5 is a bit of
stretch. While his training does give him superhuman physical powers, at that
point he would have been training for about one year. Of course an older Ranma
wouldn't be naive enough to think that Genma actually intended to give him up,
when of course Genma would just sell Ranma and then steal him back. I'm skeptical
as to whether Genma would cower before a mob of unarmed and untrained gaijin.
It's perfectly true that Genma has no taste for real danger but first you should
put him in danger. In a situation like that, Genma would be more likely to
just cross his arms and watch, calling out advice to the boy, intervening
if he thought they were actually going to kill Ranma. Bartering his son away
wouldn't be his first manuever. First he'd try polite requests, then offering
to work for passage, then promises to pay on the other side then abject
grovelling. At that he might just decide that he didn't need these jerks,
take Ranma and jump over the side to continue swimming for China.
People tend to get carried away with Genma in fanfic, portraying him as a
monster who places no value on his son. This is not true. Genma is neither
affectionate or sensible, but it seems certain that he values his son as raw
material, a chance to leave a lasting legacy. Genma is willing to do _anything_
to ensure that his son will be the greatest of all martial artists, because
at this point, that is the only thing that Genma can salvage from what appears
to be a largely wasted life. Without Ranma, Genma has nothing and is nothing.
Genma would never give up really give up Ranma, and although his judgement is
poor, he would step in if he believed that Ranma was about to die.