If I remember correctly, there is no way to NAT outbound traffic to a specific IP address. You can NAT it out to WAN1 or WAN2, but to NAT out to ip #2 on WAN 1 I don't believe is possible. Maybe there has been a feature released allowing this since the last time I had this problem (about 6 months ago) but not sure.
I would try calling Meraki support and see what they say.
That is 100% dependent on your setup. I will say though even though Meraki lacks this feature, and some other ones you would think are common, I have never hit a "there is no other way" to obtain the end result.
Contact Meraki support and see if they have a beta firmware or something you can try, otherwise you will have to figure out how to make it work within the limitations of Meraki.