As a native Dutch speaker I would disagree with what you wrote about the 'ij'-ligature. It's definitely not considered a single letter and I don't recall ever seeing it written as 'ÿ' or 'Ü'.
I'm not sure whether it's a Belgian or Dutch thing and what the context is, but I've seen it written as something very similar to ÿ (both in handwriting and in print) and I've also seen variants of a broken Ü as uppercase ligature.
I'm not saying it's particularly widespread or widely accepted, but I've seen it in real-world use by native speakers. For all I know it was just a stylistic choice or very experimental, but the same could be said about German uppercase SZ (ẞ -- vs the lowercase ß) and that one even has its own Unicode codepoint (although most people pretend it doesn't exist).
It doesn't seem too surprising either. German umlauts for example evolved a lot throughout fairly recent history (by European standards) -- we used to have a tiny superscript "e" instead of the two dots.
EDIT: To clarify: I was apparently wrong about the uppercase version (doesn't seem to have dots) but the case for ÿ seems pretty solid.
Addendum: The letters 'i' and 'j' look distinct (though very close) to me in that image. At first I thought it was just weird 'kerning', but the caption seems to say it's a single glyph. That doesn't need to mean it's a single letter though; it can merely be a ligature.
Upon further reflection, I'm not sure I understand what you mean by being thought to write them as a single letter. I was only thought cursive in school and I'm not sure what difference it would make to write them as one or two letters in cursive.
