Or solar sails. Momentum can come in the form of a photon
Also gravity assists were used in the Voyager mission.
Comfortable velocity basically means any velocity at which you can reach your destination. As space is mostly a vacuum, reaching escape velocity seems to be the biggest obstacle to getting off this rock so far.
By "junk", I do mean things like the hydrogen you see every so often. If the goal is to reach the destination eventually, as long as you are moving closer relative to the next piece of hydrogen, you will eventually get your reaction mass.
Also most people don't watch and hope for cheese. They watch hoping for a good show - some new variation on a well known strategy in the current metagame or some novel response to a build order. They want to see mind games going on without the players directly communicating with each other. None of that is information asymmetry.
Last point is, playing at anything above diamond level is exhausting. After a few games I just want to relax. Then sometimes I'll play less serious formats like custom games or 4v4 games. Other times it's good to just put on a youtube video of a game and see how pros played the build order that I or my opponent had played.
I can theorize as well. Their methodology doesn't seem much more accurate than an educated guess.
As for the topic itself, from personal experience, watching has not much to do with the additional info that the audience has, because most live streams aren't good enough at highlighting such things. The players themselves are much more attuned to the timing and rhythm of their game and even if one player can't see what the other is doing directly, he is usually more expectant of it happening than the audience can be. In many cases, he knows exactly what the other player is doing with high probability be being correct without having direct knowledge of it.
To me, watching is fun because I know how to play the game. And I know that everything the pros are doing on screen takes tons of research and practice to get right. Some are physically impossible for me to do (the speed of their clicks and key presses, for example). From playing the game, I've learned to appreciate what goes into every little action I see on screen.
A computer scientist is someone who fixes things that aren't broken.