Even birds have belly buttons, they are just less pronounced than in mammals. The developing foetus still needs a link to the yolk sac within the eggs. So even egg laying monsters will have bell buttons.
KC also stated that Egg laying species will have a short term of pregnancy where the egg will grow larger while inside the mother. After they will then lay the egg and it will develop as a nomal egg. But when an egg laying species does not become pregnant they will still lay an egg or two every so many days/weeks. Just that this egg will be about the size of a chickens egg rather than the far larger fertalised eggs.
the multiple egg laying thing has not been talked about. but species that we consider laying many eggs may only lay a few. This could be dependant on the species though, since KC mentions a pregnancy period for egg layers then it may be only 1 - 4 eggs at a time depending on the species. Compared to the few dozen to hundereds that species may normally lay.
Consider the lizardman, she may have 1 - 3 eggs at a time. Given that the egg does not develop to the size of a normal human babies head, if not a touch bigger, then it seems reasonable that they may lay 2 - 3 eggs at a time. Compared to the 12+ that a normal lizard would lay.
Then consider the arachne, spiders lay hundereds of eggs at a time. but if she were to have a term of pregnancy within her abdomen, she could probably carry 5 - 10 eggs at once.
We have to consider that compared to their original species, the eggs and babies are larger for the monster girls in comparisson.
Now considering a period of pregnancy, you can't compare monster girls to species that lay many eggs at once. Since those species do not store eggs within thier bodies before laying them. they infact produce the eggs one at a time then lay them over a period of time.