One thing that has always bothered me about toddler groups is that they NEVER take into account sleep times! Ever! There is a toddler's swimming class for LOs aged 12-24 months that I wanted to take DS to this winter and the pool time is 600-700pm! So your 18 month old eats dinner when? And then out of the pool at 700....get home, and into bed at what time? Baffling.
I only do one play group with DS and it is 930-1130 and it is for age 6months to 3yo (big gap, I know!) There are many meltdowns! The last mtg before Christmas break there were two 8ish month olds in tears and throwing total fits at about 1100 and the mothers couldn't understand why, were trying to blame one of the toddlers for taking a toy...and I'm sitting there thinking "OMG your child is TIRED!!!"
As it is, with it being 930-1130 I used to leave a bit early, usually by 1115 at the latest in order to get home and get DS into bed by 1145, that was when he was normally napping at 1130. I'd give him something to eat on the way home to keep him awake.
I think that you may find that with the activities going on, after a couple of weeks he will be able to last until 1215 without a total meltdown...but I would also be prepared for him to fall asleep in the car on the way home, if that is something you can live with?
With all that is going on with his naps, I think I am inclined to agree with your DH...it's one day a week and things aren't going well nap wise at the moment, so why toss aside a good activity because of naps, kwim?