I would like to try the CRS125-24G-1S-RM.
1. Can I have all ports other than the wan be "dumb" and operate like an unmanaged switch?
Yes, just set all the ports you wants to be part of the switch except one to be the slaves of one port, If ether1 is connected to the wan and ether2 through ether24 are to be part of your switch, just do this:
/interface ethernet
set ether3 master-port=ether2
set ether4 master-port=ether2
...
set ether24 master-port=ether2
/ip address
add interface=ether1 address=<your WAN address>
add interface=ether2 address=<your LAN address>
2. With the rb750gl I can get around 240mpbs download on a 300mpbs wan connection. Can I do better with the CRS125-24G-1S-RM?
Yes, you should have a little better as CPU frequency is 400MHz for RB750GL and 600MHz for CRS125.
3. Are there any disadvantages to using this product rather than a router without a built in switch?
In my opinion there is only advantages!