Couple of comments: If you had been in Seattle, you'd have called the game with 30 seconds remaining. They are notorious time-shavers there.
Actually, and seriously, I wouldn't feel bad about declaring 30 seconds remaining as DONE, especially since you had an unofficial stopping of the clock earlier.
The coaches deserved the forfeit. You did not cause that with the blown call; they caused it with their reaction to the blown call.
Also, in timed games, whenever one side asked for time remaining, I ALWAYS give the answer either loud enough for both sides to hear, or individually to both dugouts. Not saying you did, but the coach accused you of telling them this would be the last inning or something like that. I never use words like that without couching them in a lot of uncertainty. "7 minutes remain coach. This might be your last up unless this inning goes quickly."
As to the blown call - learn to trust your partner.