If slowing your down is your fear, I have gone to Disney with my daughter at 3,4,6,and 9 months, and we're going back in April when she'll be almost 14 months, and I really can't say she slowed us down anymore than our 6yo cousin did. My daughter's 1st trip at almost 3 months was with extended family and friends, including our 6yo cousin, 2.5yo niece, and 6 month old cousin. The 6yo was honestly more of a problem than my daughter. Yeah, I had to change diapers and her parents didn't, but I also didn't have to deal with her wanting every souvenir she saw, wanting to go back to the pool, go on a certain ride, etc. My daughter just went wherever I did and was happy, and at almost 1 she's still like that. And she has always hated her stroller, so that wasn't really an option for us. Though I should add that even though my daughter rarely naps in a stroller at home (or naps at all at home for that matter), she did it fine in Disney, I guess cause there was so much she was doing and seeing that she was exhausted and just slept when she had to.
If Quinn is a happy, friendly kid, I'm sure he'd be no trouble on the trip. I realize you've pretty much decided to leave him, but if you're still thinking about it, know that he's not gonna slow you down, since you have another young child anyway.
And if you think a young child won't enjoy it or know you're gone, I have to disagree. My daughter enjoyed every trip. Yes, even at 3 months there were things she liked-like Spectromagic. She loves characters and many rides now too. Also, she has always known when I'm gone, since she was 6 weeks old, and would cry until I came back. She was a high-need infant and had to have things perfect or she'd be very unhappy. I too am a SAHM so I'm sure that is part of it. We left her in Flounder's Reef on the
Disney cruise and she cried forever and was doing that cry/sniffle when I picked her up. Once I had her back, she was fine, so they do know the difference. I've never left her overnight since we have no family nearby and I wouldn't want to leave her with a stranger for more than a few hours. I realize that doesn't matter with you as you have your family there and Quinn is used to them so that's not a problem, but just as an FYI to others, babies know if mommy's not there.
But, no, I wouldn't call you a bad parent for leaving Quinn, but would I be able to do it, probably not. I'd just feel guilty the whole time, knowing he'd have a great time (yes, he'll be happy at home too), or seeing something I'd know he'd like and then sad he couldn't share it, all that. There's something great in taking a baby to Disney, totally magical IMO. When we watched a parade, the 6yo and 2.5yo were happy, but the look on my daughter's face when Pluto came out of the parade to pat her head was priceless. They get something out of it even that young, they just can't tell you.