J'-- Given what you're reporting with the soil pH's, acidified run-off would indicate the soil pH is pretty low,... I have had some very acidic bags of HF recently (5.0!), some others were fine,...a possible source of the acidity... also, water pH'ed correctly may be very susceptible to significant pH swings if it's very low in buffering minerals-- CA and Mg carbonates... if you use RO/DI or even some machine dispensed or bottled water, they will be very low to near zero ppm's hardness (low in buffering minerals), and as such, small inputs of acidity (or alkalinity) can cause disproportionately large changes in pH... Another of several reasons I dislike the run-off method-- too many sources of possible error and variability! The Accurate8 soil probe meter is the ticket, as it measures the actual soil pH at that level in the pot! If you can scare up the $$, they're worth every penny! (About $60)... So, let us know what your water source(s) is, if you don't have a ec meter to measure ppm's, and we can go from there.....
...the bugs sound like soil gnats!.... freakin' omnipresent, but a few are more a nuisance than an issue... easily dealt with by mosquito dunks, or similar product,.... that white stuff is more likely salt/mineral build up (maybe from the flushing?), but could be fungal,.. no red alert there either,... shoot us a pic, or scratch at it.. if feels moist, cheesy, it's fungus,... if it crumbles into dusty stuff, it's mineral salts....