Everything is inherently probabilistic. Mayweather would still have a chance in an MMA fight, but it would be much much much lower than McGregor's chances inside a boxing ring, simply due to the rule set. There are so many more things a boxer doesn't know about MMA than an MMA fighter doesn't know about boxing. A boxer has zero training in muay thai, wrestling, jiu jitsu, kickboxing, etc. They never have to defend takedowns, defend ground and pound or submission attempts from their back, defend kicks, etc. Meanwhile, mixed martial artists at least have experience throwing hands, albeit not to the same level of a boxer.
Forrest Griffin had his brain scrambled long before that final blow. He was rocked a couple of times and took some ground and pound from Silva, too.
Bigger gloves take away some punching power, but it won't be as significant as you're making it out to be. If he does manage to land, he will have more than enough power to hurt Floyd. He's got about a 20 pound advantage and many sparring partners have said he hits like a UFC middleweight (185 lb.) The only question is if his many different stances, unorthodox movement, and shots from different angles will open up opportunities while Floyd is trying to figure him out in the opening rounds. It's no different from an NFL defense giving an offense completely new looks meant to confuse them. An opponent's stance will let you read their movement and anticipate where their shots are coming from. I guarantee you Floyd has never seen a muay thai stance, a greco roman wrestling stance, or capoeira before inside the boxing ring.