Hello there. Seedlings stunting? Let's talk about it.
Firstly, ignore the high EC and high pH of your Run-off. When you flushed through that new soil you pulled a lot of soluble material out of it, and I 100% believe that's why you were seeing such high numbers in the pan. The higher 7.8 run-off is somewhat concerning, however. Our mediums will sometimes come, in the bag, @ 7.5pH, but these numbers balance out with consecutive feedings/flushes at 6.2-6.6. I know the numbers you're seeing are scary, but I highly recommend you don't flush or feed with high pH solution to try and balance it out. This will cause more problems...trust me. Just 6.2-6.6 repeatedly will work it out.
On to what I can see in your pictures and isn't' apart of your question (Kinda. It's in direct relation). When I saw you had flushed the seedling in such a large pot, I immediately feel you're now looking at an overwatering situation (which you did mention in the question). Yes, 100% overwatered. In the seedling stage there is no substantial root system to pick up all that moisture. Lot's of the moisture she needs is actually taken from the air through her leaves, feeding the root system and helping it grow. She is currently drowning and may have been drowning for awhile. This will stop growth and I do believe that's what you're seeing right now. If she remains in this state you will start to notice yellowing and other symptoms (namely Nitrogen deficiency). In the beginning when we feed the girls, especially when we start in a large pot with lots of media, I'd recommend to do localized watering .5-1 inch away from her stem with only 20-50ml of water at a time. This will effectively eliminate any further over-watering with the seedlings👍. She needs to be moist but not drenched.
As for your current position with the girls, it's time to dry them out, and with no root system to take it up, this may take a little time. I've done this!!!! more then a couple times man, and that's where this info is coming from and here's what I do/did to help it dry faster. Firstly, circulation. Seems like a give in, but get some more fans and air flowing throughout your grow area. If you can, I would purposely drop RH a little bit and raise temperatures to pull moisture out of the tent/soil. Most importantly, I till the top of my soil where I know there is no root system. Turn it all up and around 2" deep and this will seriously help you out, and dry out the medium surrounding her much, much faster. The faster it all the dries, the faster she can breath, and the faster she will grow.
Please PM me man, if you need anything further. If there are more details you think I need to know, let me know!👍 I'll end with, you'll be fine. As I mentioned, I've done this multiple times and it sucks, but after the dry out (and a little lost time😕) they do and will come back. I'd still leave nutrients out of the equation, btw. Far too early imo and this will hurt her more then it will help her, ATM. The lack of growth is not a lack of nutrients😉. Good luck with whatever actions you take in the coming day (s). Cheers my friend!