Would using timestamps solve consistency issues with snapshot isolation?
I would suggest turning on READ_COMMITTED_SNAPSHOT at the database level - that will get rid of a large amount of the concurrency issues you are seeing with the Access front end.  

Having TIMESTAMP fields in the tables that you are having issues with will help Access, but not with the ASYNC_NETWORK_IO waits.  Those waits are showing up from the various combo boxes and list boxes you have in the front-end.  Access does not request the entire dataset for those items until the combo-box or list-box etc actually needs all the data.

TIMESTAMP fields will be used by Access to determine if a record has been updated by some other process between the time Access fetched the record, and the time it (or the user) attempted to update the record.  If there is no TIMESTAMP field, Access must inspect all the fields in the dataset prior to running the update statement to see if anything has changed.