You don't really need an expensive pH meter to start out with. You can just use a drop test kit just like the one they use for pools.
You should be able to find an inexpensive TDS (total dissolved solids) meter that tests the amount of food available to the plants. It works by testing the parts per million(ppm) of dissolved solids in a water source.
I had a little stick meter that cost $40 before. It worked great, just like an expensive $100 meter.
You need some sort of TDS meter though so you can check when to feed the plants without waiting for them to show signs of nutrient deficiency. It would let you know if what you are experiencing is actually because the soil you are using is too strong for your delicate seedlings.
The pH meter isn't really necesarry, just more convienient really. The drop test kit is still 100% accurate though... a pH meter can go out of calibration on you without notice.
I would just stick with some superthrive to revive them at 1 mL per gallon of water. Only give them that strength once- after that you can add it at a drop per gallon and it should keep them healthier than without it.
The kelp is just a soil additive or a foliar spray that will give them a boost. You don't really need it, superthrive is what you need right now. I seriously doubt it's a micronutrient deficiency now that I think about it. It's either the soil is too strong, the pH is off, or they were watered too much and couldn't uptake nutrients for awhile- or a combination of the three.
Wait until the plants are established and growing strong before spraying them or feeding them a lot.
You definitely need to check for the pH as soon as possible though- that is very important at this point. I think the soil was too strong for most of them and it burned them though. Just make sure to get the pH right and not water them too much and they will be just fine
Just watch the new growth- that will let you know if you fixed the problem.