Interesting post. I've been theorycrafting a while on making a triple spaced font: 150% for the 'w' and 'm', 50% for the 'i', '.', etc.
I've never started experimenting with it, because I think it will problably look like a bad spaced and kerned proportional font.
Besides that, if you create a duo or triple spaced font, you break one of the core functionalities of a mono spaced font: it's not mono spaced anymore. For design related work this should not be such a problem. But mono spaced fonts are still widely used to generate output from programs. This can be on screen (terminal), or emails (think of a basic invoice in plain-text format) etc.
So I think whe have to stick to mono spaced or proportional fonts. Anything in between is a bad trade off in functionality. But, as an expression of a certain taste (more precise – the love for the rugged and awkward look of mono spaced type), duo or triple spaced font may be viable in limited scenario's.
I was mostly referring to 'ﬁ' etcetera, the one's like 'æ' I have less trouble with, because of correct spelling as you pointed out wisely. I don't have a 200% width axemple at the moment, neither do I have a solution.
For me ligatures like 'æ' are a real big issue in a monospaced font. They need to be in the the font, but at the same time there's not enough room for them. In my (semi condensed) mono spaced font in development I've omitted them – until I've found a solution.
Double acutes are okay-ish, because I'm used to them. The single acute on the ij-glyph (top line) is viable, and I like it, but this one is a matter of taste. Singe acute on i-j combination (bottom line) looks like a mistake or error to me...