I feel a complete failure

You are absolutely not a failure.
I feel maybe your DS is feeling very stressed with his life atm and is taking it out on you both.
You seem to do so much for your grandchildren and your DS will soon regret his actions but may be too stubborn to admit this right away.
Obviously your concern is for your grandchildren right now and not being able to see them. However, I would suggest leaving it for a bit so your DS can realise quite how much quality time you’ve given your grandchildren and what a positive impact you have on them. Possibly part of the problem is that they get less one on one time at home and they want it, expect it and don’t settle for them as easily as they might once have.
I would also suggest never commenting on how they are parented at home and no references as to how many children are in their blended family. That might be taken as a slight against their parenting skills.
When you do get access again, I’d also say it might be good to tell them directly any information they are asking your grandchildren about. Say that you are putting DGS2 to bed as asked but that it seems he doesn’t fall off to sleep initially- ask what they suggest you do to help this (even though you know that what you have been doing worked so well).
I don’t think it’ll be long before you get asked to help out again but I’m also wondering if you should perhaps not be so available.
I know it’ll be hard for you in the meantime as you both obviously enjoy your time with the grandkids and they love being with you but perhaps while this is all being worked through, do something special for yourselves on the days you would’ve had the grandchildren. Just so the day doesn’t feel so empty.
But remember you are not a failure.

The boys are their children so it is their rules. Although I do have sympathy with your situation you come across as critical of the parents and their way of living and I expect they know how you feel.

You say that they quiz the 15-year-old as to what goes on, are they not just checking that you are following their rules?

You are used as a taxi, have given them money and dropped everything to babysit. Doing those things was up to you and should have been done without strings, it does not mean that you get to have the boys when you want to.

The boys are old enough to have contact with you independently if they want to, I am assuming that they have their own phones.

Hmmmmm. I wonder if grandchildren love coming and there is jealousy. Or..more simply, their lives are so complicated and chaotic that working it all out including your good selves it seemed easier to say no.

I think nanna's advice is good, but it will hurt and I do understand this.

Although I think nanna8 is right, I feel for you. You'll suffer badly, worrying about your 2 GC missing you. Sounds like you're the only stability they have.

I think you are doing way too much for them and not doing their parents any favours. They are not being responsible and they are just over reliant on your good will. Maybe if you are out of their lives for a little while your son might realise just how much you are actually doing ? I’d leave them to it for a while.