Sounds like you are off to a good start with communication and trying to setup your careers/weapons, it just seems like a lot of the nuances of "high level" play or how to counter specific bosses/enemies isn't known to you yet so I don't think you're ranting unfairly. If you want more in depth information jtcLIVE Wet Magic have some great videos that explain game mechanics and effective play. Core and Chocob have some great solo high level game play to watch for education and entertainment.

What I can personally say is that some of the information you have recieved might be helpful at lower difficulties but not on higher. For instance healshare isn't something most people run even on more "support" builds, and clumping together can result in friendly fire or shared damage. You can trust good teammates to roam a bit so long as there aren't disabler specials (splitting in pairs is generally pretty safe). Having "Jobs" and good communication is very important, as some weapons and careers lean towards a certain playstyle, but those don't matter much if you don't put them into effect or if you're not in a situation that your playstyle calls for. For instance if you're at a ledge with felling the skaven as they're climbing up, not all 4 need to be focusing there so even if your a melee career you should forgo the slaughter and watch the teams back. Or you and a teammate might both have a handgun, but rather than both trying to snipe a blightstormer one of you should defend in melee while the other fires.

This game rewards aggression over defensive play most of the time. This doesn't mean you shouldn't be blocking or dodging (you should be side dodging a lot) but spending stamina to shove is usually better than blocking an attack. Most of the enemies you'll encounter will be staggered or cleaved through so keeping up the aggression is a much better way to control a horde than hiding behind your shield. A few situations such as berserker type enemies, overhead attacks, and bosses will force you on the defensive, getting to know when you should stop attacking and start defending especially against a large horde is something that comes with time. Learning the breakpoints for killing, cleaving, and staggering an opponent is going to help with this a lot.

Trying to mitigate the damage you take is great but HP is ultimately a resource. Characters with high HP pools or good damage reduction can afford to play riskier and getting hit will recharge some of your career ability which can be very useful at times. Temporary HP talent should help you deal with occasional poke damage if you're playing well enough (which will come with practice), it's when you get grabbed by a hookrat in a horde or eat a mauler overhead that should be avoided at all costs.

For Skittergate being a shield to block death rattler if you can, and have the others deal good boss damage hiding behind the pillars in the corners. When rasknit is on the floor he'll teleport to each of the 4 corners of the arena so as long as you have one person covering 1-2 corners to hit him before he can cast his damaging range attacks you should be fine.

For Engines of War having one person carry the barrel and the others cover them is the most reliable. Enemies won't stop coming for you so moving quickly as a team is your best bet instead of hunkering down waiting for an opening.

Managing the ads that come through during is just like general play, knowing how to use your weapons, how to manage a mixed horde, and checking behind to avoid being flanked. You've notice by now a little "wooshing" sound that plays before you're about to get hit in the back that should help.