Oh my gosh, Fluid Engine is worse than I feared. I ended up biting the bullet and putting a few hours into learning it yesterday, and find out it's incredibly buggy when it comes to how it displays a page on a smart phone.
Example: it's very common for me to put an element (e.g. a social link) at the bottom of a section, only for it to appear (on a phone) at the top of the section. (Does this also appear in the "mobile view" preview? Yes. Can I edit in the "mobile view" preview? No, confusingly.)
Another example: there's a long, empty space in one of the sections (about 1 screen tall) that appears in mobile view. Can I get it out? Nope.
I trust that SS will fix these problems. In the meantime, it'd be nice if I could make a good-looking website using the old SS.
Squarespace employees who might be reading this: you're putting your product at risk. You're gambling with your company.