Hi,
I am by no means a css/html expert, but I did find the solution to your 2nd question.
https://community.shopify.com/c/Shopify-APIs-SDKs/Product-image-issue-when-embedding-Shopify-Buy-Button-on-3rd/m-p/826203#M53654
Here (especially on page 2) someone explains in detail how to change the html url file. You open the css of your Shopify button (which you've pasted into squarespace), then you see a url - open this in a new tab and copy paste the whole thing into word or pages. Cntrl + f and look for 280 - change 280 in 1024 or whatever you want it to be (I changed all four of them into 2048px).
Save the whole document as a .js file. Then go to Squarespace, create a new page (disable it), and create a text block and type a word, whatever word. Link it and instead of linking it to a url, upload the new .js file you just created. Add it to the word you just typed, copy the path which is displayed /s... something. And then go to the page settings and go to advanced. Type: <script src="PATH"></script> (customize the PATH with the path that you've just copied from the link file). Save and go back to your Shopify buy button css.
Replace the url which you've used in the beginning to obtain the text for the JS document with the new path you've still got copied - the /s/PATH.js thing. Then save and voila, you're popup images should finally look like the way people intended photos to look in the 21st century.
Again, I'm no expert on this, just followed everything they said in the link shared above. But it did help me out of this misery. Perhaps it will help you as well.