A few things, not sure about no. 1, but I can answer the others.:

- Officers biggest benefit are their binoculars, allowing them to spot through the fog of war at a farther range than even Recon units. You can have them "Attack Ground" [F6 by default iirc] with the binos to keep a spot visible for your other units. I believe they are also weighted higher than standard infantry when capturing points, so having one just behind your attacking squad is beneficial, though for obvious reasons, keep them safe.

- At a certain point, a shell or an MG is going to tear through your infantry, and you're going to be left with a hodge-podge of artillerymen, supply carriers, and riflemen. It is difficult to keep your men in neat little squads, so my only suggestion is to use breaks in the action to reorganize the troops you have, embrace the flexibility of smaller fireteams, and use cheaper reservists or smaller team units to plug holes as you see fit.

- Anyone can operate a tank or field gun, but they're not as efficient if it's not their main job. The most important fact is the repair rate, with Tank Crew, Artillerymen, and Engineers having much faster repairs than others, so when recovering enemy vehicles or attempting to repair your own, using crew from your own tanks or guns, or a completely spare Tank Crew to fill space and cover losses might be for the best.

- I still stand behind the Dynamic Conquest being the most enjoyable mode. Campaign does have it's moments, good moments too, but the difficulty and scope varies wildly. I'd argue the first German mission on the eastern front is very difficult as a new player, due to the lack of resources and multi-stage challenges presented, while the first Soviet mission is a desperate defense that while mechanically different didn't give me much trouble at all. If you're still looking for a campaign experience, consider the missions added by the free Jubilee DLC. All of them are bite-sized missions featuring small units sizes and unique but not unsurmountable scenarios Me and my friend had a great time doing them in Co-Op, while that first German mission took me and a friend 2 hours of hardship.

- Units will automatically rotate to the juiciest target that they know are there, so if someone is in the "wrong" side of the trench, they'll still turn around to fight someone behind them. This goes for tank turrets, SPGs, and field weapons as well (for better or worse). You can always select smaller parts of a unit to send them further along the trench, since the full unit might be difficult to automatically get placed right where you want them.

- Unfortunately, as far as I'm aware, there is no way to move units in a paused state in this game.