First off, for the record I'm a presenting author at this year's 100YSS. I don't handle the tough stuff (quantum mechanics, warp drives, particle shielding) but I'm versed in it. In other words, I'm no rocket scientist. My focus is on encouraging pre-teens and students to pursue careers in astronautics and outreach.
That said, it's the very first thing you say that I have to take to task, "The scales you're talking about with interstellar travel are almost humanly unimaginable."
I do not speak for anyone besides myself when I say, bunk. The scales we are talking is the nearest star to earth, Proxima Centauri, is 4.2 light years away. That's traveling at approximately 186,000 per second for 4.2 years. That's imaginable. It's difficult (and arguably impossible) but it is not anywhere near unimaginable.
So maybe you meant that the means to achieve such speed over such a length of time is unimaginable. I'm here to say, again, no. There's a popular joke around theorists of interstellar travel, "Physicists have no problem with getting us to another star system. The hang-up is with the mechanical engineers." (Which is actually pretty funny.) The point is it's easy to conceive of ways to manage interstellar travel. The challenge is it's (presently) impossible to build them. But the ideas, plans, and models are there, for sure.
So with all that in mind, 100 Year Starship-as-a-program has set up to get something to another star system. We just don't know what or how. But the beauty of the concept is that Jules Verne wrote and published From Earth To The Moon in 1865; roughly 100 years later we went to the moon. Vis-à-vis this model, 100 Year Starship has begun as a thinking consortium or brain trust to engage some of the best minds in their respective fields with the challenge of reaching a nearby star 100 years from now.
(Incidentally, last year I believe Dave Neyland made a comment the "laughably slow" comment brought. Referencing the fact that technological breakthrough continues happening even after you set a plan in motion—ie old tech on spacecraft compared to what is currently available, it's what we had when said spacecraft's program was started and outlined—another joke, "If you leave on a starship and the starship that leaves after you passes you, you're on the wrong ship". Another techy joke. But an eyebrow-raiser also.)
Lastly, "barring someone radically overturning Einstein", heck, even I can imagine that. And we don't have to overturn Einstein, just pass him on the outside. I say this to kids I work with all the time, "You can never prepare to be surprised." We don't know how we are doing what we are talking about doing but not talking about it is sure to not get us anywhere.
Remember, a talent is someone who hits a target no one else can. A genius is someone who hits a target no one else sees.
Which naturally explains what I am doing being an astronaut teacher