OP, you mentioned escorting them to their rooms, is that roughly getting them off to their rooms, no idea what exactly this means but if it is physical 'cajoling', please avoid it. They will be bigger and stronger and this will not work so please resort to logic, clear instruction and negotiation where necessary. Eight is too old to need to escort them and ten (IMHO) definitely is. My dd at 12 is now much stronger than me!
"I just feel bad because of threatening to ring their dad like it was a punishment. I am always dreading the day they say they want to live with him, or spend more time there than they already do, yet am acutely aware that it's their right if they want it."
Please stop offering/threatening more time with their dad, especially if you feel he is not a brilliant influence on them. You will make the thing you fear most more likely to happen, IMHO. Their dad should be neither a threat (I do understand why you said it but still) nor a treat.
The fact the children feel they spend less time with you may be because their time is full of everyday stuff. So maybe make the time that is spare, the family time, more positive if you can.
We have got Headbanz game and my kids (6 and 12) will happily play it (we got it from a charity shop for £1.50!). We also play lots of other board games and short family games like blow football with cotton balls, or mini basket ball with cotton balls using the other persons arms as the basket ball net!
These help to make family time better.
If you notice that things are hard when your kids come back from your ex's make sure the arrival back time can be a time of doing nice calm things and a sort of re-connecting.
Sorry if I sound preachy but I've had lots of advice on family dynamics as our son is adopted and we have had a lot of issues with dd, who has autistic tendencies.
If you have time to read up the Parenting Puzzle is fabulous. I did the course with it when dd was 8 and I was really struggling. DD was sometimes threatening to run away etc, being physically a bit violent or rather aggressive etc!
"I feel like I guilt-tripped them tonight. I also feel ds1 feels more responsible for things than he should."
Then move on, make sure you do not do things that you could perceive as guilt tripping (e.g. making them choose between you and their dad) and make sure your sons feel appreciated. EG the spilling of the beads was your fault, you missed the table. You could have thanked your older son and told him he did not need to demand a 'thank you' from your younger son.
BUT I really thought this would be a lot worse so sounds like you are doing a fabulous job, especially parenting alone. But if you feel you need to do somethings differently then you can make the change. Make sure you look after you too!