Lets say the Database server is 10.10.10.1 and the web engine is 10.10.10.2
Database access for FM Go and FM Pro is handled by the Database Server. Users would direct the apps to the IP/DNS name of the Database server in the example 10.10.10.1
Webdirect users access the databases from the web engine. Users would direct their browser to the IP/DNS name of the web engine in the example 10.10.10.2
2 - Not sure on this one other than App DB connections require a certificate be added to the Database Server
3 - Yes you should be good to go with the versions of clients you mentioned with the understanding that the features of each client will be limited to items that the particular FMP version was able to do and not features from later versions.
4 - Email? However you notify users that systems will be down now.