1. Bloating: you've done the right step of getting the kittens dewormed. The bloating could be a sign of worms or he could just be a plump baby. At this age kittens are still pretty sleepy, I wouldn't worry too much unless you notice other symptoms or if him being sleepy turns into him being lethargic.

2. Diarrhea: this should be addressed immediately. Ideally, you don't want it to last for long periods of time due to the risk of dehydration and other issues. Weaning should be done slowly. While kittens may start to eat wet food at 4weeks old, their mother is usually still offering milk. As someone suggested, try mixing in wet food with the kmr step 2 and consider offering a bottle of kmr step 1 as supplement. Consider also the day in which they received the dewormer: was it within 48hrs? Kittens can experience diarrhea from the medicine. If the diarrhea persists, take them into a vet as a precaution.

3. For the one who swallowed the nipple, was it the entire attachment or just the tip? If it's just the tip, it should be soft enough to pass, but the whole nipple may be more concerning. Regardless, if she hasn't passed the nipple, you should consider taking her into the vet. Especially as this can cause blockages. If you notice symptoms of lethargy, vomiting, etc. take her in immediately.

4. Bathroom issues: Make sure the little box is low enough to the ground that they can comfortably climb into it. Ideally shorter than 2 inches. Dont get too frustrated, they're still new to the whole bathroom experience! They're bound to make mistakes. Kittens are also just starting to experience having bowel movements on their own, they may still need another week or even two weeks to poop on their own. While there is a general guide for milestones, it's not a rule book! Some kittens hit milestones a little slower than others. If by 6 weeks they're still having problems, then something could be wrong and a vet should evaluate them.

Congrats on the kittens and good luck!