1) no major difference, whether you put in topsoil or compost or
anything in between.
2) no problem whatsoever with the sulfur, since you are talking about
hundreds of grams
over a season. The compost you already put in will buffer that and much
more. You will
grow excellent garlic and onions.

You got off to a very good start. to make it better, you ought to have
the soil analyzed and
if you find that it is deficient, add amendments.
My personal instinct as far as soil is concerned is to put in as much
compost as possible,
plants that grow in it tend to be very strong. In fact, I tend to be
creative and use a lot
of dead leaves and wood chips long before they are compost. They
provide mulch,
which is always a good thing to have in a garden. Mulch plus drip
virtually eliminate
weeding and watering, which are the non-harvesting chores in the main
season.