Hi,
first of all - next time please create a new thread to avoid discussing several issues on top of each other. Makes it easier for people to search for the right information later on instead of looong threads 🙂
For your question; I think you will have problems with this design as ERPS relies on defining east/west physical interfaces and link-down on each of these will trigger a notification for the other members of the ring. With MC-LAG you create a aggregated ethernet link with at least two physical ports between your QFX switches to avoid have the inter-chassis-link down at any time. This design works against the ERPS design.
Furthermore ERPS is not supported on QFX5000 series or EX4600 in virtual chassis - so doing a virtual chassis per site and then ERPS between the VC's won't be an option either. I suggest this isn't supported due to the architecture with a Junos RE VM running on-top of the physical hardware with a risk of too high latency in ERPS keep alive/notifications.
Only viable design I can think of with the proposed hardware and physical design, would be to have layer3 links betweens each site and then do your stretched layer2 via EVPN-VXLAN. Requires licenses on the QFX5110's and quite more configuration on each switch.. but it would do the job.