[girasquid]

You already accepted the offer from the second company. It's clear from the speed that the first company has been responding to your emails that they're either disorganized or not very interested in you - neither is a good thing for a potential employer.

In your shoes, I'd tell them that I'm grateful for the opportunity but have taken another position due to the delay and leave it at that.

[existencebox]

Thanks; that's both what I've been trying to tell myself and what my other half has been telling me, but it's easier to believe when a third party says it.