Spatial awareness, yes. The engineer can help with this respect when choosing a base site. It varies from map to map, but sometimes it’s about controlling the angles you can be approached from; and fired from. It’s also about delegating responsibility as a team - obviously there’s no point in everyone watching the same approach and neglecting other approaches.
Depends on the team. From an engineering perspective, if you have a good engineer then it’s usually rare that they need extra time to repair stuff. Engineers should use the intermission to repair stuff further away from the base, and then work their way back to base. Any fortifications in your base or just outside, can be repaired during the wave. Engineers should start further out and work their way in. It saves time and makes more efficient use of time. Equally there’s a philosophy that if an engineer is struggling to keep up with repairs within the normal alloted time, then they’ve probably built too much.
I’d also add that some classes should be prioritised for the final kill - the Infiltrator with the Reaper card for example, gets stim for every shotgun kill; execution build Nomads knock off time from their ultimate recharge and get stim if they’re running Menace and Execution Shield. Veteran’s Brutal Efficiency speeds up their ultimate; and precision kills for the Robotics Expert (with Precision Repairs) provides repairs for all fortifications. Sometimes it’s not so much time, but being mindful of the needs of team mates.
I try to keep an open mind about using different skill card builds. Sometimes it’s good to step out of the standard meta and you’ll be surprised at how effective some of them can be. I mean, some people insist that the Demolitions MUST have a Lancer GL and insist on the Custom Lancer GL card, but actually master Horde is totally doable without the GL at all. Rifle-builds for the Demolitions and Tactician can be pretty effective as well.
Sometimes the game doesn’t always highlight downed team mates. I’m not sure why it is. Other times, it’s down to lack of spatial awareness on the player’s part. Decision making around revives can be a difficult one at times. Obviously as you say, if there’s a tiny chance of them getting shot and downed then they should take it. I rarely see this happen though. What I normally see, is players who are helpful, but reckless who run out to get a dangerous revive, and end up getting gunned down as a result.
Sometimes it’s due to players not marking targets or noticing them. In a true team situation where people use mics and are in party chat, it’s easy to coordinate. With randomers, it’s a bit harder to predict. I’ve sometimes marked a dangerous enemy, only for EVERYONE to turn their attention to that enemy, only to let in a few smaller and individually less dangerous enemies as a result. It can get a bit silly really. I’d also say that part of this, is that some players struggle with their role and struggle to deal with certain dangerous enemies. For example if you have a Demolitions on your team, then the easiest and quickest way to deal with Guardians and Sentinels is to mark them and use your ultimate artillery strike, but sometimes players rush it and launch it as it is moving and miss it altogether. Some classes are more efficient with dealing with certain enemies than others.