..did I ask if you have calibrated your ph meter recently, just checking,....
... slurry test is from any left over soil as mentioned, not from your pot because indeed that would shred the poor things!
...straight run-off isn't a reliable indicator of in-pot pH at all,... at best, if it's very close to input pH and that's in OK range, it's close enough,.. if it's waaaay off, then you know at least that (the right) serious corrections have to be made,...
>>> that pH is too high for starters mate, true soil pH needs to be around 6.3-6.5,...300ppm is hard-ass water! .... too much Ca can interferes with uptake of other nutes, but CaCO3 is not that soluble in water, it's needs acidity to breakdown, so while it does offer pH buffering toward neutral pH, it does not provide much actual free Ca++ for uptake... So flushing isn't going to fix the overload of CaCO3,... it will however strip the soil of the other nutes, hence the likely cause of the yellowing,... never flush such "water only" type soils, for this reason,....
Taproot hitting the bottom doesn't mean squat! I'm not sure what the store dewd was driving at, but that's irrelevant,... for example, have you seen the 200 fabric pots used for huge plants? Shaped like a kiddie pool, taproot doesn't get to dive deep, and it doesn't need to,...as long as there's plenty of room for feeder roots to do their job, it's fine.... In that case shop guy is right, the soil got tapped out in that 1gal pot....
.... check the chart here, if you soil pH is off enough either way, P availability will suffer....
--- adding Ca-Mg to the distilled is good-

..... leaves falling off mean they are tapped out, if they were yellowed out, necrotic,.. needs pics of those anyway,....
.... don't use hard core pH solutions to try and force the pH down, this is traumatic to the roots,.. that's it still coming out alkaline confirms that there's serious deposits of CaCO3 in there, the acidity is eating away at it, but then again, that's how the buffering works, it dissolves freeing up the ions to react (mainly forming bicarbonate HCO3- from CO3--)... meantime the roots are saying--

..... I'd use the PBP because it has a more complete nute profile, and use the bat poo as a PK booster, weak dilutions....
.... best bet: use low ppm water plus Ca-Mg (readily available ions, minimal carbonate), pH to 6.0 if needed (may not be with nutes in there), and ride this out,.. more flushing and flogging may just finish the roots off and that fucks the whole dog then! it does look like you helped it some with those last readings,...