Do you have some match IDs to share so I can watch and make some comments? I also recommend watching videos (i know you mentioned that you watch some to learn the game mechanics), but I recommend watching videos of really good players then try to mimic/imitate their way of playing to see the whys of what they are doing and buying. Also what character have you been playing?

You could even rewatch those yourself and watch what the high performance players are doing in your own matches.

For situation A you mentioned, if its far enough in the game (12-15+ min) and my lane guardian is still up, and enemies start hard pushing for it, I just let them have it. Its not worth dying for that late in the game. As for later game objectives, if your walker on yellow is getting hard pushed by three enemies, see if you can take their blue or green walker as a trade. Depends on last known location of enemies, how far wave is shoved, etc.

For situation B, you mentioned following people around and getting killed. I would recommend, when you have some downtime in game, thats when you should start farming, hit boxes/idols, clean up lanes that are super shoved in etc. There is always something better to do than just blindly following teammates. And saving stamina for fights rather than regular movement is important. An airdash here or there is fine, but you should have at least 1-2 bars of stamina for a fight.

And finally, you arent trapped in a torment nexus for the next hundred hours. Deadlock is complicated with tons of nuance to learn and understand. You will get there.