Re: RSTP : Edge port with received BPDU > 0 ?

I might suggest you look at this thread below;

http://forums.juniper.net/t5/Ethernet-Switching/MSTP-with-Juniper-and-Cisco-Switch-for-load-balancing/m-p/301185#M15281

Different products, but you are in same boat.  I would suggest you VC the 2 x EX4500 and get rid of STP from Core to Access.

Now as for edge-port, see below for how this all works.  I am going to assume you have not configured block-on-edge.

https://www.juniper.net/techpubs/en_US/junos15.1/topics/example/spanning-trees-bpdu-protection-ex-series.html

Good luck.  Yes I know it will be difficult for you to now move away from STP Core to VC, but maybe worth the effort.  In hindsight you should have gone VC Day 1.