Holy crap this is one hell of a bug report! Thank you so much, this is really really helpful!
I'll admit that I haven't been able to test the game on Linux myself, so the build is a little blind - you're the first to report these issues, though, so I'm really really thankful. Hopefully this means I'll be able to fix these problems for others users with similar machines! I'll try to address all your issues one by one:
"1. Windowed mode doesn't seem to work."
That's odd. The most recent build should have support for resizing windows, but either way, I don't recommend it - it's an early fix and can easily mess up the aspect ratio of the game. Currently looking into fixing that!
5. I've tried to scale the window down... (This is connected to the previous question/answer!)
I guess that's a good time to comment on the resizing with the dialogue text - the game is very hard-coded right now and will natively run in 1080p - I wouldn't be surprised if that's what's causing issues for you considering your monitor outputs lower than that. That ALSO means that the font is rendered at a set size, which means it doesn't follow resizing of the window. (Like I said - very hard-coded) I'm currently working on a fix for this, but it's causing a lot of bugs which is frustrating. Hopefully I'll be able to have a fully functional resizeable window by the time I release on steam - if not, I might remove that option for now but add in the ability to change resolution.
4. When I had no save files and went to "Load", there was - unsurprisingly - nothing there. When I came back to the menu, "Save" button was gone. I don't think that was an intentional practical joke? ;)
A little confused by this one, sorry - was there an issue with saving and loading? You're never able to save in the main menu, but both saving and loading should be available in-game!
(Untitled Clam Location bug question - "Oh, by the way. You see where Clam is? I've talked to Tilda...")
That's a known bug and currently fixed in editor! It'll be updated with the next release, which may or may not be later today! Thanks for reporting it either way - it's not game-breaking, but it is very bothersome. Turns out I wasn't expecting people to save and reload into that scene, which was silly of me!
UI AND NON-BUG REPORTS (Sorry about the disjointed order of responses)
Question #2 and #3 are really really good comments - it's very little work for me to add in a little tutorial thing on the main menu.
The thing with the corner UI - this is locked by design during the intro of the game. I don't want people to be confused or distracted by it, so it's locked away not only then but also during any conversation! (There's even a joke about this in the intro dialogue) That said, your comment regarding the meaning and purpose of those buttons is fair; the idea is that the player naturally will be curious and click the buttons as soon as they can, and will be able to check what they mean by doing so. I'll have a look today at possibly including text that hovers over them saying what they are, or even adding them to the tutorial thingy.
I think that was everything - I'm real sorry to hear the game has such difficulties running on your machine! That's the first real technical issue I've seen, as the game seems to run fine even on 6-7 year old laptops. I'm guessing it has to do with the set render of the game, but I'll have a look and tinker around with it.
Out of curiosity, did the game run better/worse in browser mode?
Let me know if there's anything else I can help with. Huge thanks for all this - this has got to be the best bug report I've seen so far!