Well, I do like Jum, so much so that I've more or less quit going to any other islands, at least as long as it lasts. Jum is being hooked up to the power grid and we're afraid the Koh Jum we know & love may
have its days numbered.
To address the OP, there is a public longtail from Laem Kruat at around 16:00. You should not have any problem making that. We asked New Bungalow to ask the boatman, Man, to pick us up at the airport, which is actually quite close to Laem Kruat. We'll see if that works out; it's the first time we've ever tried it that way. If you take a taxi from the airport to Laem Kruat, I'm not sure how much it might cost, but the amount of 600 baht springs to mind.
As for #4, you obviously didn't see much of Jum as Tezza says. It has beautiful beaches, rocky coves, stupendous landscapes, lush forest and a steep mountain, a perfect Thai fishing village, all kinds of different accommodation at all different price levels, great eateries, even a few beach bars, and lots of friendly, warm people. What it's not is a party island. If that's what you want, go somewhere else.
Personally I'm a big fan of New Bungalow. Along with its next door neighbour Joy, it has arguably the best stretch of beach on the island. It's got great bungalows at reasonable prices, and the food is superb.