Need directions for the easiest way to separate the PCs behind the RB/SXT bridge.
They need to use same gateway, and the same mail server in the LAN but to not be visible from the other side of the bridge.
RB/SXT bridge can also be connected to the main router Mikrotik RB493AH
Any network setup (reconnection) can be made..
make a vlan to managment Your Wireless Devices. Put the 88.0/24 to the vlan Interface, and tag to the swithc. tag at Wireless Devices, and put the IP to the vlan Interface. Dont route trough the vlan in the dualwan router.
Its also possible to do a vpn to the dual wan router. More than one solution to this. 
I am not clear with your first suggestion?
Had a mistake with a previous setup.. Have a linksys e3200 router instead of one switch.