FWIW: you have to be affiliated with an academic institution to get access to most full texts on Hathi. However, one such academic institution is the Library of Congress. I would think anyone reading Hacker News in the US would qualify as a researcher at the Library of Congress; the catch (admittedly a big one) is that you have to present yourself in person to get a library card ("reader registration card" in LoC parlance) -- meaning that unless you live in the immediate DC area you have to travel there. And they're only valid for two years.
It’s a closed-stack library and their catalog is online. Figure out the items you want to start with before you arrive so that you can put in the order; it takes a little while for the staff to fetch it from the stacks and deliver it to the reading room.
Also, pick your reading room carefully: the general collection can be delivered anywhere, but there are are some items that you can only get in a specific one.
It may be the only way to find some of thing things preserved via citizen archives. I enjoyed the talk Ian MacKaye gave there about archiving the live performances of Minor Threat & Fugazi. Here [1] is a post on the subject.
Hmm, that seems unlikely but I honestly don’t know. Possibly free for US citizens but a small fee for visitors? I can’t imagine it would be closed to visitors, though you should be able to contact them to find out.
As I recall the Library of Congress also has some digital subscriptions on-site, though not Elsevier. One subscription the Library of Congress has that Libgen and Scihub don't have is ProQuest, so you can download theses and dissertations from ProQuest there. I'm not sure that you even need to be registered to use the building wifi, actually, though I'd recommend that too.
Another way to access Hathi is by physically visiting an affiliated university campus and doing it from their computers. Most university libraries allow the public this access.
As you seem to know stuff about the LoC: I've read that of each newly published book in the US you have to send samples to the LoC. However, I've read that they don't keep all samples. Do you know what happens with the ones they discard with? How many are discarded and what are the criteria?
"Each working day the Library receives some 15,000 items and adds more than 10,000 items to its collections. Materials are acquired as Copyright deposits and through gift, purchase, other government agencies (state, local and federal), Cataloging in Publication (a pre-publication arrangement with publishers) and exchange with libraries in the United States and abroad. Items not selected for the collections or other internal purposes are used in the Library’s national and international exchange programs. Through these exchanges the Library acquires material that would not be available otherwise. The remaining items are made available to other federal agencies and are then available for donation to educational institutions, public bodies and nonprofit tax-exempt organizations in the United States."
I swung by the LoC on a biz trip to DC and obtaining the reader registration card was a quick and painless process; highly recommend to anyone on this forum who finds themselves in the area and can spare an hour.
Link to how to get a library card at the LoC:
https://www.loc.gov/rr/readerregistration.html