Not read the whole thread so apologies if I'm repeating anything.
Personally, I think it's time to let go when the bun stops fighting. As long as they're still showing a will to live (nomming away, coming up to say hello, refusing meds etc) I'd keep going.
I have had two buns with EC, Vinnie, the most recent, was battling it from November to April and had several courses of treatment, she improved then rolled back again once we stopped. She died in April, I found her in her favourite hiding place so I think she knew she was going and went to her special spot. She fought right to the end, loved her basil and was a menace to get panacur into.
Anyway, sorry, going on a bit. Her story with description of her fight is on page five of the link in my sig. As long as the bun shows that they want to go on, I'd stick with it. People may call you cruel but you know what's best for your bun, Vin had to wear nappies for a short period and one of my friends went nuts about it, but she's never had rabbits so what the heck would she know. You'll know when it's time, you know your bun. Best of luck with it and lots of vibes.