1. I notice that several Nedor comics were renewed on time on the 28th year which one would assume would make them protected under copyright, but I'm guessing the is no copyright holder or else every comic book company that is using them would have been sued by now. Can anyone explain this situation ?
The common wisdom is that the renewed copyrights were never transferred anywhere. So they're still under copyright technically, but nobody has grounds for an infringement case. Whether that's actualy true or not, though, I couldn't tell you.
2. I have heard many Charlton comics are in the public domain because of lack of a proper copyright notice. What does a proper notice look like or what was incorrect about Charlton's notices ?
A well-formed copyright notice is what you've seen most of your life. It's the word "copyright" (or the circled-c), the year, and the name of the copyright holder. "Copyright 1940 XYZ Corporation," for example.
Charlton, as befits their other main business publishing sheet music, was more likely in the '60s to (wrongly) use the ASCAP statement that you see on your CDs. I don't have anything on hand, but it includes terminology like "International copyright secured," which...well, there's no such thing as an "international copyright." (I do know what ASCAP means, here, but it's long, boring, and not relevant to comic books.)
3. I know that only some of the Fawcett comics were renewed, but why are the non-public domain titles on the site ?
Serj, who runs the site, decided to host them as a quiet protest to the Captain Marvel/Superman case.
Additionally, it's possible that the books also fall into the Nedor category, since it's very uncommon for companies too actually buy copyrights to old material as "the rights."
4. How do you know if international comics such as ones for the UK or Canada are public domain in the United States ?
There are so many treaties and contradictory laws, I'd just assume "no." European countries have a Life+70 copyright term, and have retroactively instituted the terms every time they were extended. What this means is that everybody who might have owned a part of the copyright needs to have died in 1939 for the book to be in the public domain...which is unlikely, given when the books would have been published.
Annoyingly, the Life+70 rule is relatively recent. It was, previously, Life+50 in most of Europe. That means that quite a few Golden Age books in England WERE in the public domain in the '90s, but ceased to be so when the new law rolled out.
5. Characters such as Plastic Man, Blackhawk, Captain Marvel, and some the MLJ heroes while in the public domain are trademarked. What restrictions would this put on them and how can you get around it to use the characters ?
The way I find easiest to think about trademarks is by taking the word literally: It's somebody's "mark in trade," or their corporate identity. As long as you don't imply a connection to or endorsement by the company owning the trademark, you're safe. Generally, that's going to mean keeping the trademarks out of any material used to sell, like a cover or title. (I'm sure there's some amount of boilerplate disclaimers that can be spread around liberally in the "Blah is a registered trademark of XYZ Corp., and its use suggests no..." kind of vein.