Playing with VRFs - what am I doing wrong?

Yep, so - essentially - we are still (back) in the “you’d better use a different network, or you could …, but …”.

BTW mtest001 has already posted another tested approach using (or mis-using) proxy-arp here:

http://forum.mikrotik.com/t/failover-between-2-isps-using-gateways-with-same-ip-was-nat-traffic-to-vrf/170381/1

this proving that VRF’s are not strictly necessary to solve this problem, in his words making the Mikrotik a “kind of L2 router”, but that I prefer to consider a “cleverly configured” switch.