A bit. "hobby" should be pronounced more like 'hobbi', and the double b does sound strange indeed.
Does "hobby" ever change form in Polish, or does it stay the same always?
the noun stays the same. Adjective is hobbistyczny and acts like all adjectives.
Geography is his hobby is not acceptable???
The subject is different. In "Jego hobby to geografia", 'geography' is a new piece of information, not something you knew from the beginning.