Any particular reason you want to buy the passes online? I thought the prices are pretty much set. We usually just go into a brick and mortar discount travel agency to buy them and get them on the spot. The only quirk though is that I think because of the rise in the CDN dollar, the US pricing is cheaper since pricing isn't updated regularly and the place we go to is "obligated" to sell us at CDN pricing. (They could be BS'ing us tho. )

Just a throw some comments at your original questions...
We usually have just reserved on overnight trains and high speed/specialty trains. Never had any issues finding a set of seats together (though we usually travel during shoulder season).

We find railpasses convenient but you only get bang for your buck if you take longer trips. A strategy is to buy a shorter pass and just buy point to point tickets for shorter legs.
There are also occasional added freebies and/or passholder discounts associated with the passes. We once took advantage of a nice free cruise on a lake with our pass.