I did not read your whole [long] text, BUT I believe the better way to to this is to get rid of STP altogeather. STP based designs went away 10 years ago. Or at least 5+ -
Instead take the 2 x EX4300, make them into a VC. Now these switches act as 1. Then create FEC from any of your Cisco access switches with the links split between the 2 x EX4300. On the EX4300 you just create an AE (LAG) with a member physically residing on each switch. Then you get completely redundnancy and active-active forwarding on all links of the LAG/FEC, without the need for any messy STP configuration of any type.
Welcome to 2016 and better way to design networks!