It may also be worth noting that the illustration of Maxim's 1963 Helvetica comes from a wonderful book on the work of Moscow lettering artists. Like a number of other examples in the book, this is a typeface that was never manufactured. Experimentation in such circumstances is cheap: it never has to undergo the rigours of making something that can be used and facing uncertain adoption in a market. There's a lot to be said for such experimentation, and one of the things I love about that book is the optimism it presents with regard to what might be possible, which ironically flourished in the Brezhnev period when so little was actually possible.
In his open letter to Vladimir Yefimov, Maxim quoted Mayakovsky: 'I love our designs’ grandeur', noting the tendency in the Soviet period to judge the success of a design by its intention rather than its achievement. We risk doing the same thing if we elevate experimentation as an end in itself.
Could I for instance use m instead of т or n instead of п without trouble?
Depends on the style of type and how consistent you are. There's no reason why an upright and formalised version of the cursive letter shape norms can't constitute a valid style (it is the norm in Bulgaria), but I would say that you should try to be consistent, so that readers familiar with the cursive shapes will engage with the style on those terms. Mixing and matching typical upright letter shapes with cursive letter shapes would be too Frankenstein, I think.
For traditional text faces, I use a simple rule: x-height + descender = cap height. Ascender height is basically stylistic, i.e. it can vary pretty freely depending on the style of type and the effect one wants to achieve in text. But the relationship of x-height and descender height to cap height is foundational. Interestingly, I didn't start from this idea, but realised it after noting that my inverted exclamation mark always fit perfectly into the x-height and ascender: I was doing it intuitively.
Although it's an unpopular position, I tend towards the double period. My reasoning is that if the abbreviated word occurred in the middle of a sentence, I would expect it to carry a comma or other clause-level punctuation in addition to the period indicating the abbreviation. So why shouldn't this hold true at the end of the sentence also? Inded, it does, if the sentence ends with a question mark or an exclamation mark. So why not if it ends in a period?
At root, gender's just a fancy word for category, so I just assume that categorising typefaces by gender makes about as much or little sense as other classification schemes; i.e. do it if it is useful to you in some situation, but don't imagine that it describes a reality external to the classification.