1) What are the US Garrison counters for?
Garrison markers can be placed in an unoccupied German position that is in range of an engineer. See 19 and 19.3 for engineer capabilities.
2) There are US Inf counters from both divisions marked by their div number.
No landing space or beach designation.
When and where do they come in? Which rule explains it?
Units without a turn of entry are placed in beach landing boxes during setup. See 2.2 The playing pieces.
Please note that I have first edition of the game. These rules should be the same but the sections may be a bit different.
Have fun, it's a great game!