You just need to search for papers, quite a few available.

In fact, during the 60's Algol variants were already being used to write compilers and OSes.

That's true. Not only compilers, but a lot of MCP was written in Algol.

I'm sure they did. I'd even expect most compilers were written in a HLL. Note that most refers to quantity: I expect most compilers were written as an academic exercise with no real intent to be used in production. I would expect the compilers that were intended for people to use to write production code were written in assembler because at the time compiler optimizations where not very good; computers were slow and expensive; and programmers were cheap: as such it would be worth the extra cost to write your compiler in a low level language.