Did you have a separate thermometer/hygrometer in the incubator? For some reason, the ones that are included are notorious for not measuring temperature and humidity at all accurately. If only 3/9 developed at all, that's the likeliest culprit. Next would be the source of the eggs.
If you're on day 21 with chicken eggs, I'd be inclined to leave them in the incubator for a day or two longer. 21 days is the norm, but if the thermometer was inaccurate and temp was low, it's not abnormal for them to take a bit longer. It appears that you originally set 9 eggs and 7 have...
For some reason, both on this post and the one about button quail, I'm seeing black boxes with "This video is unavailable" while looking here on the forum. I haven't changed anything on my end and used to be able to see videos directly. Odd.
Edit to add: I went to a different browser and it...
If your incubator decent view windows, you might be able to get a cheap web cam and at least be able to check on the eggs while you're not there. With a large enough SD card, you could probably even record the hatch. It's not as good as seeing it live, but it's better than nothing.
I used the Wynola Ranch cages and ended up putting a small paver brick in each cage to give them something off wire. They couldn't move it around and I think it helped keep their talons and beaks better. Something like this...
The only real differences between a UPS and "solar generator" tend to be battery capacity, battery chemistry and the ability of the latter to charge directly from solar panels. Most UPS I've seen use Sealed Lead Acid (SLA) batteries which are less suited to full discharge (typically only good...
If you're running electricity to the shed and only need coverage for power outages, I'd suggest a small computer Uninteruptible Power Supply (UPS). A small 650 VA UPS should run a 150 watt heater for over two hours, maybe three. Those are relatively inexpensive, too.
If you end up having to use a different watering solution, I have the 250W and 500W versions of this:
https://www.amazon.com/dp/B0D73CSZBW
I haven't tried either, yet, since I took a break from quail, but my plan was to have a thermostat to turn it on when necessary. Something like this could...
I just looked at the labels on the Mazuri Gamebird feeds. Except for the price (on Chewy), the starter (over $1/pound) and breeder (83 cents/pound) both looked pretty good with 30% and 20% protein respectively. The breeder with 2.5 to 3.5% calcium sounds good for the hens, but I'd be concerned...
If the plastic is too brittle to cut, you might be able to melt it with a cheap soldering iron. I wouldn't use a good iron on it, but there are plenty available for under $20.
I'm not sure why you would have been told that they would die with a high protein feed.
Quail need a high protein game bird feed. The 8 week old should probably still be on a 28%, or better, starter and transition to something a bit lower, 20 - 24%, at around 14 weeks, which is what they should...
Sorry for the loss, but in that case, it was probably for the best. Good to focus on the 8 healthy ones. Are there more eggs in the incubator, or is that it?
I assume the vendor will be sending the correct eggs at no additional cost to you? If you haven't started incubating the chicken eggs and aren't prepared to have chicken, have you considered eating them? If you want to incubate them, then list them on Craigslist, that would also be an option...
It certainly looks like a chicken egg to me. It might even be a duck egg. I found this image on Reddit that compares from top to bottom
Goose, peacock, turkey, duck, chicken, guinea, quail