AFT fluid in coolant issue

The transmission cooler on my Civic Hybrid (CVT) croaked in a big way, causing the transmission and coolant to mix freely. The issue became apparent when the coolant overflow tank spewed all over the engine - and there turned out to be buckets full of ATF in it. I got a new radiator, and had the tranny fluid replaced to clean out the transmission (put 24 quarts of transmission fluid through it before it finally ran clear, then changed it twice more, just to be safe - seems to have worked, the tranny's working.)

So I went for a long drive yesterday on the highway, and noticed at the end of the day that I smelled coolant. Took a look, and, surprise - there's ATF in the overflow tank, again. There's no sign that the radiator install was bad, so my worry is that the bad coolant (pre-repair) drained back from the overflow tank into the main body of the radiator, then found its way into some engine dead spaces - like the water pump.

How likely is this? If it's likely to be the case, how the heck do I get this crap out of my engine? I know they ran a ton of clean water through - both before and after changing the radiator, so where the heck is this stuff hiding, and how do I eliminate it.

Just have to keep flushing it. It can be a nightmare. No secret tricks that I know of.

Thanks! That's what I'll do.