Modification is defined exactly the same in the AGPL as in the GPL. I don't think putting a reverse proxy in front of an AGPL app means much of anything in terms of either the letter or the spirit of the license.
Mongo's separation of Apache-licensed drivers from the server is about making it so that applications don't have to link to the AGPL'd server code, thereby avoiding "modifying" it. Without a modification, the network interaction section isn't triggered at all.