Well, just in case someone else has a similar issue sometime down the road, I'm posting to say that the problem is solved - sure enough, it was a faulty EI. Took me about 3mins to change out the sensor, wrap the SPE EI in foam and strap it in on top of the CRRC Pro EI; kicking myself for not trying this sooner cuz it took much longer to clean out the carb.

I'm sure other experiences may differ, but I've had rotten luck with these Chinese EIs - My first was a SPE 26 EI that worked flawlessly through testing and setup then died the day before its maiden flight (a blessing in disguise, I guess). This CRRC Pro EI died within 10 tanks on the ground.

But the replacement SPE26 EI has been pretty good; and it seems to work fine on the CRRC Pro engine. Haven't fiddled with the needles much, but even at 1.5 turns out on both needles and the factory timing with the old EI, idle seems fine and top end sounds good! Got my fingers crossed.... gonna break out the tach and start tuning next.

Thanks to all who offered help!