If you found eggs and want to raise them, go for it. I have released monarchs in October with no problem. The way I see it, they will grow in the luxury of our homes, instead of outside in the heat/cold/rain/wind, so they are very strong when they emerge and can take on just about anything. If left outside, they will probably never make it, simply due to other obstacles that they face.
And, there are lots of reasons monarchs don't make it. Don't beat yourself up about it. If you have two that are lethargic, don't seem to be progressing or growing to full size, it's nature. The main thing with raising them is to keep their food supply fresh. Baby cats can eat on the same leaves (again, I usually cut off a stem with 3-6 leaves on it) for 4-5 days, as long as the leaves are in water and stay fresh. The larger cats can easily go through 2 full leaves (I'm talking common milkweed, with leaves that are large) in a day each! I'm having a heck of a time keeping the cats separate with their own food source. The only reason I'm doing this is because I want the ones hatched on common milkweed to stay on that, not switch, otherwise I have to keep track on what I need to bring inside.
Surprise ... well to me it was ... I have 3 baby cats now! There must have been an egg I was not aware of on one of the leaves, so now I have an extra baby! My number is up to 11 - 4 in chrysalis, one in "J" form tonight, so will be chrysalis tomorrow, 3 tiny babies (they sure grow slow the first few days) one medium sized cat and another large cat. I also have released one monarch over a month ago. So, my year has been very late this year! Last year, I had released probably 20 by this time, and was working on my second group, for a total of 42 released.
I just figure, any I raise and release are that many more that I know made it at least to butterfly stage.
Next year I want to grow host plants for black swallowtail butterflies - dill, fennel, rue, queen anne's lace to name a few. I tired this year, but weeds took over the area. Next year I will plant these things in areas when I can keep a closer watch on weeds.