I think that trying to hold is your problem. From what I've found, that is definitely the spot to start, and the spot to try to get back to, but it is difficult to hold for an extended period of time. Instead what we do is we hold it until it is becoming unsafe, then we jump out of the back of the boat onto the bus. We run from there to the spawn point (let's call this run Stretch 1), along the big open area and go up the stairs to the right of the overturned car (Stretch 2). Then we go straight from there into that tunnel that loops around in the shape of a U(Stretch 3), and in the tunnel we'll call Stretch 4. From there you run back to the hold (Stretch 5) and hold until it gets unsafe again. The good thing about this kite is that you have somewhere to hold for a short while to clear a large amount of zeds, unlike some other kite paths on other maps where it will take longer because you don't get a rest. If you need some screenshots, let me know.
It doesn't matter too much along Stretch 1 if you get split up a bit, as long as you don't get blocked when trying to get to the spawn point. From there, along Stretch 2, again getting split up doesn't matter too much as the spawn points for the zeds are at a distance. However, when going up those stairs and into the tunnel (Stretch 3), you want to all be very quick and close together, because zeds can easily get between you on the left at the beginning of the tunnel. Through the tunnel (Stretch 4) you'll want at least one person, probably the leading zerk/firebug to sprint to the end as fast as possible to stumble/panic any big zeds that might be trying to block you off. From there, getting back to the hold is often not too troublesome, as you can usually run around any blockages due to there being two lanes. However, zeds can become a very big problem if you're slow along Stretch 5 as there a multiple entry points along the sides and in front. Stretch 2 is a place where the zerk can engage a raged big zed for a short time to give the others a bit of breathing room, and maybe the others can help to finish it off. As long as you don't take too long with it, otherwise Stretch 3 & 4 will become blocked. That's the general rundown of the path with problem points noted. Now for the team composition.
If you can field a full 6 players for your team, then you'd probably want a zerk, firebug, medic, commando, demo, and then maybe a support or sharpshooter. It really depends how much you're struggling along the kite path. If you're struggling a lot, a sharpshooter won't be overly useful. A support is alright at everything and has the advantage of being able to function as a backup medic (with more than just a med pistol). The zerk and firebug would be there to clear a path in front and make sure to prevent big zeds from hitting your teammates. The commando can clear trash (particularly crawlers and stalkers), can carry a med pistol unless it's a strange loadout, and is really great for extending zed times, where medics can sedate, zerks can clear a huge amount of trash, and demo can nuke. A support with supplier would also be handy for the firebug, as they get a full magazine back for the mwg. The demo would be the primary big zed killer, and could also clear clumps of zeds with an m79. I noticed that I haven't included gunslinger at all, and I guess that that's because there are several points along this kite path where a gunslinger would be pretty useless, and the other points could be better handled with a demo or sharpshooter.
I don't think running multiple supports is overly beneficial. They can't quickly kill big zeds unless very skilled, and only if there isn't trash crowding around. They're also not great at clearing trash in some circumstances, such as a carpet of crawlers around the feet. They also can't pick off medium zeds at a distance without off-perk weapons, where you might be better off going as that perk instead. I'm not saying support is bad, not at all, but I don't think having more than one is usually required or a good idea. Also, I would've thought that the entry points to that spot are a bit too open for supports.
I'm not sure if you're happy to kite or not. I know there's plenty of people who don't like it, but it is really fun on this map.
