He seems ok but needs fed ASAP when the crop is so flat like that.
The last chick has a very full crop. You want the crop full, but not bulging. So if you have to step in and handfeed, fill it a little less than that last picture. If in doubt, feed a little less. You will have to feed the bird again slightly earlier but it is better than accidently overfeeding and then stretching the crop. Weight the chick on an empty crop in the morning, then feed 10% of that chicks weight for that day.
The crops should have something of food in all through the day. Let it reduce down, then feed when there's just a little in there before it empties completely. Then once per 24 hours the crop should be allowed to completely empty like the crop of the first chick picture above. Best time for this is overnight.
The next day, reweigh the bird and adjust the feeding quantities for the day as the bird will now weigh more and so there feeding will be ever so slightly more.
You can feed a little over the 10% if you feel the crop looks quite flat or not full and the chick is still wanting food, but be mindful of not overfeeding.
I hate to sound like a moron
You're not a moron. A moron would sit back and leave the birds to fend for themselves at whatever cost. You are here and trying your best to care for them.
To put your mind at ease a little, when eggs are laid, the hens usually start to incubate after a couple of eggs. Meaning those first chicks hatch very close together, and then there's a day gap more or less between the others. So the elder chicks often looks more similar to each others size over the newer chicks size.
Another contributing factor is by the time the little ones hatch, the elder chicks are bigger and will take the first food from the hen. If she is not experienced, she could easily be feeding the little one less than they should be getting because the elders are eating everything they can. Another reason to keep a close eye, and supplement some food where needed.
Please keep posting if there's anything else you're wanting to know. Formula prep etc.