I think we addressed the first issue before. A MS Access Project database ONLY contains Forms, Reports, Queries, Modules, etc. But Not tables. The tables are on the SQL server. You can't access the adp file and pass through to the SQL server from a remote machine. You need to copy the database front end to each computer that wants to use it. Each computer must also be able to "see" the SQL server - or have the proper DSN. Does that help?
As for 2, it is possible to do a payroll database in Access. How many employees? If there are not too many, this is easy. I would also probably take lots of back ups of the database, maybe one each pay period. I wouldn't exceed about 2,000 employees.
To plan it you need to first know what you want to get out of it (reports) and then what you are going to get to put into it (data entry,) and then plan the algorithm to get from input to output. That middle bit is the hard part. How much help do you need?