Help!!! Newbie in need of help!

Alright I need some advice. My plant in flowering is yellowing horribly. Older sun leaves are dieing like normal but most of the plant is yellow especially near the terminal buds. The leaves are supple and adhered to the stem. Some tips are brown and curling down. They are not easy to pinch off but are very yellow. This is my first grow and I must admit to not being the most consistent. I have read so much that it could be toxicity or deficiency or pests or soil. I live in a rural area and do not have access to quality soil. I have been using MG Organic Choice. When I transplanted her into a 4.5 gal pot I used only the MG Organic soil with no amendments like perlite added. I realize this is a mistake. Plants since then have had 1/3 perlite added. My designated grower overwatered them (daily) for about a week and fungus gnats developed. I used sticky paper to collect adults but had to order BT var. israelensis to kill the larva. I just applied it yesterday but have not seen an adult in about a week. I flushed the plant approxiamately a week ago as well. It is under a 600w HPS, with CO2 enrichment. I was feeding FF Big Bloom, Carb Load, FF Green Stay, and Bud Blaster. But have not fed in about a week. Before that I fed at every watering (same as grower I recieved strain from). My friend who grows this same strain says to give a couple Veg fert feedings (FF Grow Big) to up the N level. I am 4 weeks into flower and growth has slowed significantly. Ph in water is 6.7 and soil is right at 7. Temps are between 80 and 85 in day and 69-77 during night. When transplanting I did not disturb the root ball, just repotted. I cannot afford a proper pH electrode and have been using a cheap soil probe to test pH. I'm sure all of these problems are compounding each other but what should I do. Transplant w/ more perlite? Buy proper pH electrode? Fertilize w/ veg fert? Flush more? Other nutes? Foliar feed w/ high N fert? Thanks so much for the help. I have solicited help from every source possible. Being in a rural area I have a great need for a community like this. Thank you.

I'll hand out rep like candy for legitimate answers!!! lol

Hey Jesse,
Man, Far as I can see your plants look down right great. I'm 'fraid the yellow tint toward your pictures hides even the faintest trace of any kind of yellowing. Outside of the "yellowing, like I said they appear in fantastic shape.

Thanks I appreciate the feedback. The very top leaves are so yellow, like canary yellow but have not started to die. They remain supple with good stems. The picture is misleading.