Synthetic Backup using BE2015

Synthetic backups are supported only for file system data and not VMs.

And regarding encryption, you can always restore from a different media server. Prior to the restore, just create new encryption keys using the same passphrase.

Definitely, not a recommended way to backup. You can enable GRT in the Virtual-based backup and then one single backup gives you either complete VM recovery or a single-file restore options.

File / Folder GRT restores does temporarily stage the entire VM on the media server if restoring from tape. Not from disk. Secondly, if backing up to tape, you are essentially using up more space than a single VM based backup as you are backing up twice.

Also, A synthetic backup is used only if one does not run recurring full backups. In other words, only the first backup is a Full backup and all the remaining backups are all incrementals.

If you really need to run Agent-based backups in addition to the Virtual-based ones, no need for the synthetic feature.

So can't I do a full file restore with synthetic full and some incrementals, without the normal full backup available?

Yes, you can.