As long as you configure the ports on your MS120, in the same way as your 2690 switch (i.e. trunk/access port type, vlan, etc) I see no reason as to why it should'nt work. Assuming that all Layer 3 functionality is on the branch MX, the MS120 will only act in Layer 2.
Remember that Meraki is Cloud based, so all configuration of the MS120 switch will be done on the Meraki Dashboard, contrary to your Catalyst 2960 switch.
In regards to the CAPWAP tunnel from Branch A to Branch B, you might need to look into Forwarding ports udp/5246 and udp/5247 to the WLC, assuming you are routing the CAPWAP tunnel over the internet (which I wouldn't recommend). If you have a VPN tunnel between MX at Branch A and ISR4400 at Branch B, you should be ok.
LinkedIn :::
https://blog.rhbirkelund.dk/Like what you see? - Give a Kudo ## Did it answer your question? - Mark it as a Solution
🙂All code examples are provided as is. Responsibility for Code execution lies solely your own.