IBackup for Synology app is currently not available, you will have to map the NAS and select data for backup from the mapped drive.
IDrive does incremental backups. What this means is that it backups only the data/files that have changed since last backup.
If data is encrypted by crypto virus, and if IDrive has done a backup of that data, it is unlikely that there is any recent version of that data on our servers as the data is usually not editable by user's applications and nothing would have changed since the encryption.
Also cryto-virus typically infects all or most of the files. What this means is that if you want to restore data from IDrive through desktop client, you can get the data back if you just restore - 1 version. You can make use of folder wise restore option.
To know more about folder wise restore, please refer the following link:
https://www.idrive.com/online-backup-classic-faq#folder