No, EA is not part of the interrupt control logic I meant, and I would like other contributants to confirm this:
The mechanism I meant is the one that allows only one ISR to run on one of the two priority levels at once. (you can't nest interrupts that have the same priority, that's what the levels are for).
The processor "knows" that it is doing an ISR and on which level. Hence the special RETI instruction. It's this "knowing" which I meant with the mechanism. If you don't execute an appropriate amount of RETIs, the processor will keep "thinking" it's running an ISR, and will not accept any more interrupts.
The EA flag never gets affected through all this.
I know you have your reservations, but check the tutorial on RETI.