Mangle based on in-interface and out-interface

RouterOS Beginner Basics
1

When using Masquerade why dont you

chain=srcnat action=masquerade dst-address=0.0.0.0/0 out-interface=ether1 log=no log-prefix=""
chain=srcnat action=masquerade dst-address=0.0.0.0/0 out-interface=ether2 log=no log-prefix=""

This should cover what you wanting to do and then you would just need to configure your Mangle rules to track the connections both incoming and outgoing.
Try not to confuse your nat and your mangle.

2

No, in-interface=ether3 does not cover vlans on ether3.