Oh yeah - I had major childhood trauma concerning the "a":
https://listserv.heanet.ie/cgi-bin/wa?A2=ind9910&L=typo-l&P=R1336&I=-3

Mixing:
The various combos are pretty common, except for having a mono "a" but a bi "g" - that seems very rare.
What's "correct"? Depends. But generally bi-s are like spinach: funny taste, but good for you.

Germans:
May god help them: http://news.bbc.co.uk/2/hi/europe/3943507.stm

hhp

1. No. I have no memory at all of my first consciousness of the difference between the single- and double-storey a and g. I have no memory of ever having a problem with the distinction. However, I learned to read quite late (around seven years old), and I suspect most of the books I competently read first used the double-storey versions. By the way, there's something to be said for learning to read after one has developed a reasonable vocabulary: one gets to start with much more interesting books.

2. Yes. I think there are very few non-italic typefaces in which the single-storey a looks good, while the either version of the g can be used much more liberally. Frutiger is a good example of a typeface that very successfully matches the double-storey a with the single-storey g, but there are very many others.

1) I remember loosely that I thought of printed letters as doing things that handwriting couldn't and shouldn't. It (the bi-level 'a' and 'g') just seemed like another weird no-reason rule of Life that I needed to know and not question. I was a very passive kid. However, it did seem to make a little sense, since I remember thinking the same thing about serifs.

when I was very young I used to make triple and quadruple story E's. (Basically a vertical line with as many horizontal lines attached as I felt neccessary that day) ahh, typography was so much fun those days