I already said THROW THE RED PLUGS AWAY! They will essentially suffocate your embryo's by diminishing how much air flow goes through the bator. No, getting rid of the plugs will NOT negatively impact any of the eggs or embryos.
No fan.. IF temp on the bator is accurate.. I would be surprised to see anything hatch by day 21... they *should* hatch late in theory according to your temperature. With no fan.. ... temp should be set at 101.5. But see what day your chicks actually hatch on since this is where we are at now.. and adjust your future temps IF needed. You would very likely see the fan if there was one inside the top. I didn't think I could make one out. So if you don't think there is one.. there likely isn't.. it would have had to have been added after market on that bator... but so many folks do that so figured asking was the best option.
No problem... we all start our adventure somewhere and answering questions when I can, is my way of sharing in your adventure! Plus it gives me opportunity to review and challenge my thought process to never stop learning and improving.
40-50% humidity was perfect for the first 18 days. Did you see your turner working during that time?
Have you taken out the turner now... candle the eggs and draw a line around the air sack. Lay them down on their side with the lowest mark on your air sack facing upwards... AND raise your humidity... probably filling ALL the wells in your bator. Aiming for 65-75% humidity.
If you are able post a couple of photos.. one you can see all is good and the other you say is too full to see anything.. and see what out experienced candlers make of the embryonic development. The differences may be hot or cool spots inside the bator causing developmental delays or boosts. Or it could be normal ones and quitters. Without seeing what you're actually looking at, too hard for me to give any thought.
Yes, some variation in temperature throughout the day can be very normal. Within moderation it isn't too much of an issue.