Since you used salty sand in all liklihood, it could be part of the problem. It could be the nutrient itself being poor, or it could be the pH of the medium itself or some other factor. What it is not, is a problem with your tapwater & feed pH, unless there's a serious problem with either of the two. However before your grow was all green, so it's not the tapwater itself IMO.
1) I don't think so. Get neem seed cake and sprinkle on top as a top dressing.
2) Ignore run off pH. a) it's meaningless (IMO, for what it's worth) and b) making adjustments based on this figure will most likely make the problem worst, or at best do nothing.
3) No!
Take some cuttings and start growing in 100% pure canna coco professional. Use Hesi coco nutrients and measure pH
after adding the feed to 5.8-6. This is about as simple as you can get, and removes all the variables in your current grow that could be contributing to the problem.
Growing in coco is hydroponic growing (passively, not active) and in veg growth rates are like comparing a rocket to a beat up old reliant robin car.
I think in future you need to simplify your grow as much as possible and then once you've got the hang of the basics, start experimenting with different products and techniques. If you were to go with the Canna coco pro and the Hesi coco nutrients with a pH and EC meter I guarantee I could guide you through right to the end of the grow with no problems like this whatsoever. Continue with your current grow, but consider doing this.
Alternatively go with a simpler, tried and tested soilless mix with fewer fancy, exotic and impressive sounding ingredients and just use fishmix and maybe some mollasses and seaweed extract occasionally if you want a bit extra 'kick'. But just think, if you ditched all the complicated products that supposedly would give you a higher yield and just made it as simple as possible, as described above, your yield would probably end up (ironically) being higher in the first place.
This is a lesson many new growers learn - keep it as simple as possible and listen to the people with experience in real gardening as well as cannabis growing and you'll likely reap the fruits of your labor.
Edit: I just wanted to say to you, PsychicBubble, that the only stupid question when it comes to growing, is the one that is not asked. Nobody (me least of all) will judge you for making what might seem like schoolboy error. So fire away with anything you'd like to ask about with respect to cannabis and particularly cultivation, I really don't mind. If you're ever embarrassed to ask a question because you assume it's too simple, please don't hesitate to send a PM if you must. Otherwise just ask in the grow thread and I'll be more than happy to accoodate all of your questions, no matter how simple or complex they are

You do need to put your faith in me though, and if I say negative things about your medium, or the fertilizer you're using, please do make the effort to, at the very least, to experiment with other individual clones. Remember they are all clones fro the same plant, and comparing the effec of doing x, y, z are so much easier if they're in the same environment and you're only changingone variable at a tie.