If you are really carefull on links, content and theme quality the drop with be temporary. But complete structure change with different headings, changed internal link structure can really do harm on your site for a bit longer.

As long as gross kept the same only the theme changes, and you don't build of a lot of 404's you'll be fine within 1 or 2 weeks.

Uhmn. Yes and no.

302 suggests it will be back. but actually it will be never back. 
So a 302 would keep the link juice where it is which is fine. because you do not want to have all that juice to the invaluable login page right.

but noindex following is the way to go.

a 302 would suggest the page will be back anytime soon since its a "temporary redirect"
While a 301 suggest the page is moved. So personally i would use a 302 so that the page your landing on keeps the juice but the user will be redirected to a login screen for example.

I would suggest to noindex those pages if they are really hidden for search engines. (noindex,follow)

Interesting reads:

• Save your website with redirect on Moz.com
  and - SEO Advice discussing 302 redirect on mattcutts.com

Yes indeed, if its really a usability thing and you really help people with good content, and extra specific content under a read more that perfectly fine. But in deed be aware of the amount. If you reality punt a small line in preview and pages and pages under a read more that wouldn't be advised. but generally no worries!

You are already answering it yourself a little. iFrames are really bad practice and can through-off search engine bots. If you say the content in the frames are all the same a few alarm-bells go off as well. Why would you want to use frames anyways? maybe you can clarify that a little or show us a the example so we can make the advice a little specific what to do next.