Im sure wordpress like joomla has an addon for url rewrites essentially 301 is the way to go...

Get a plugin to make it easy

I think it should be one way or the other.... Especially if you want returning visitors they might not notice the first time but after a while people are bound to notice

I think if you have very thin content and the source code is the bulk of the content lets say 80% source code 20% unique google may not look favorably on that....  Google has algorithms that put cookie cutter sites in a lower position than custom sites so it is important to have your code somewhat unique.

But only have 2 of the sites the same probably is not "the" problem it mainly to penalize people who buy these money making sites for $500 that look the same as 200 other sites except for the content

Domain authority is slightly like an average page rank on your whole site try to build up quality backlinks on pages that have a low page authority as well as continue to build domain popularity on social sites