To start things off, I'm not a lawyer thus this is purely speculation for discussion. There are a few attorneys who roam the forum but you are always best getting your own counsel for legal advice, especially when it concerns IP where a slight differance in words means a whole lot.
Trademark law is long, complex and has many details. I've studied bits of it but can't even start to recreate it here.
Trademarks protect names but it's not absolute. Prior usage negates strength and of course an overly generic name just can't be protected -- unless it's famous. Fame [within a niche] is difficult to establish but grants far more power and is what you see with companies like Apple and Disney.
As far as using a mark to stop theft of your name. That's an interesting approach but it won't be simple. Keeping the name locked with a safe registrar is far more prudent than trying to sue someone if they take your name. That said, adding a trademark to a name significantly increases your sale value as you are offering that strength too.
There's been a lot of cases over domains and they've shaken up trademarks significantly from the days where names could be local [region use to be a major factor in determining marks]. The TLD is not a factor in the name and does not impact the trademark although with the growth of TLD types who knows if that will change.