I've been asked about this a couple of times today. Here are my thoughts that I have shared about it.
If others were to obtain other domain names, Trumpcoin.us, Trumpcoin.online ect. could they be used to be redirected to the main site?
Yes. I do it all the time to lots of sites. I had 2 domains parked at Trumpcoin.rocks for about a month.
Would this even be something worth while?
It depends on the domain name.
Worth while to the Trumpcoin.rocks devs? Not really, unless they like your domain better than theirs and decide to promote and index it.
Worth while to you? Maybe. You could do your own indexing (SEO) and optimize better than the target site, which would make your domain more valuable. You could even break off into your own site later and take the bookmark traffic with you.
Domain names are like coins. The original owner pays less than $20 and sells it to the highest bidder when the time is right. You can make thousands easy. Millions if you hit a good niche or bubble.