.... that bugs looks like a springtail,... poke at it, and if it teleports away, it is! Not a problem bug,....BUT check the plants for odd looking leaf damage, just to be sure,.. certain species of thrips can look similar, though I don't see any damage myself in these pics,...
I'm stumped on the Mg issue, given what you've done so far,... Mg does not lock out unless it's waaaay off pH, no other signs of that,... too much Ca can interfere with Mg uptake theoretically, but without a soil pH, it's a guess at best,.. too much Ca would also show high soil pH,... foliar should circumvent this in any case,... interveinal chlorosis on lowers is almost always Mg defc. only,.. on uppers, it might be a micronute,.. but with it showing both places, I'm not sure,... How long have you been foliar spraying, and soil dosing? Seems enough time has passed to show corrections,..? They seem to be growing OK, so no worries about a face-first turn here,... About micronutes, Earth Juice Microblast is a great product, so it might be worth a try to treat soil and foliar with that,.. are you using a wetting agent when you foliar? without it, the effectiveness is greatly reduced,... it's about surface area covered, especially leaf bottoms where most of the stomata are,.. that's the way in, and w/o it, most stuff just beads up and sits uselessly on the surface,... I like Coco-Wet for this,....