Is using FastTrack at MT router with NAT overrides(disables) Simple Queue for traffic passing that router. In other words - can I use FastTrack AND Simple Queues?
Yes, but only if you have an "accept" rule in the firewall to accept the traffic that needs to flow through the simple queue (with established,related setting), and place it directly before the fasttrack rule that would otherwise fasttrack the traffic. That way you can continue to fasttrack all traffic except the traffic that you need to run through a simple queue.
If you want to run all traffic through simple queues this will not be worth doing because then there won't be anything left to fasttrack after you have made all of the accept rules.