Take all advice with a grain of salt and do what makes the most sense.. The most logical sense. Everything you're feeling/experiencing is because this is all new to you. I personally don't cull or kill off plants.. How can you learn what the problems are and how to fix them, if you kill off the seedlings with issues? You need as much experience as you can get at this stage.
"Flushing" water only soil does pretty much nothing.. You can't "flush" nutes out of organic soil. The nutrients are still in the form of organic material. The purpose of "flushing" is to correct ph issues and get rid of excess salt build up caused by months of using bottled nutes. Neither should be an issue with water only soils.
You will want to only water the base of the seedling until the leaves have reached the sides of the pot. Otherwise you risk overwatering.
You are seeing slow growth because the seedlings are in 3 gallon pots. Once the roots have hit the bottom of the pot (about the same time the leaves reach the edges of the pot) you will start seeing the daily or rapid growth. All its doing right now is building a root zone or foundation. At this stage you should be seeing a new set of leaves every 7 days or so. So at two weeks above soil, the seedlings should be starting their 2nd set of leaves.
P.s.. Rule 14 of growing your own... "Master your patience".. It's a must, lol. It will save you countless hours of worry.
My verdict? The seedling without true leaves may not make it.. But it may! Who knows, lol! The one with tip burn should be fine.. It's just more sensitive than the others and should grow out of it.. I don't think you need to make any changes or do anything specific at this point! Your soil is not to hot for autos.. (That's kind of a myth from when autos were still new) if it were, all four seedling would have tip burn.
I hope everyone's answers have helped to ease your mind a bit! I think your plants are fine so far, (except for the one without leaves) and I think you are doing fine so far! Deeeeeeeeepp breath innnnnnnn... Now exxxxxhhhaaallle! Lol. You got this!