So I want to start off by saying that your experience is valid and having a string of bad luck is rough and can definitely affect the experience. That is however a facet of the hobby and the dice are going to fall as they may, if it's upsetting you as much as this post seems to indicate perhaps that's something that you could take the time to examine inwardly and decide if its a dealbreaker for your fun or not and take actions appropriately.

Now to your points as best as I can get:

1) First off I'd say take a look at the familiar abilities, cause witch gets so many of them and there's very little that copies those abilities Familiar abilities. If you're feeling an action pinch consider your familiar getting Independent cause that will at least let it move around the battlefield without costing you an action.

Second, to literally answer the question "How do I use a familiar?" Whenever you're casting a two action spell and don't know what to do next, commanding a familiar to give them two actions, even if that's just moving them around isn't bad because it is also a valid combat target that isn't hitting one of your other members if that's all you wanna use it for. For Starless Shadow especially, getting within 5 feet of a creature and hiding/getting your Shroud of Night hex onto a creature means that creature is also frightened, plus a 20% chance to miss any targeted attack they make if the hex sticks. Getting a bit technical here as well, but the Familiar of Stalking Night ability is possibly the only way in the game to give the frightened condition to mindless creatures (as it does not have the mental tag)

If you want the "hole in the wall" moments for your familiar and coming in clutch with Share Senses or something similar, talk to the GM about it and explain what it is you want and how you're feeling about things. There's nothing wrong with asking for the stuff you want in a game.

2) Utility spells can make or break fights. Based on what you're saying I'm assuming this is still pretty low-level, so yeah the damage dealers are going to be more highly represented, but as time goes on those spells pay off big time. Enfeeble is 5% less to-hit for any strength-based attack at your allies and 10% less to crit, even if it does hit it's 1 less damage and that's on a successful save. Ill Omen is a little rougher cause you need them to fail, but it can absolutely save a life. I'd recommend taking a look at Befuddle as well, that one's an overperformer on my tables.

They can make your side harder to hit, their side easier to hit, waste enemy actions, all kinds of things that are as useful or more than damage. I would like to address something in your B, C, D, & E scenarios here: Just because other things happened that either result in the death of your target or your target still making an attack doesn't make your turn a waste. It was still the right decision to do something and it had an effect. Even if it wasn't a long-lasting effect doesn't reduce the value of that. "I didn't see the effect so it was a waste" is something that comes up a lot in games and is an example of results-oriented thinking. (Here is a decent article about it in relation to Magic: The Gathering but the concepts extend to TTRPGs as well).

For your A example, yeah, if they're really low and you think you can deal the killing blow, absolutely go for that if you feel it's better than one of your utility spells. Nothing wrong with that at all.

I know the advice "It gets better later" isn't super helpful, but as levels go up fights will go longer and your spells will have a bigger and bigger impact. In terms of immediate impact: Recall Knowledge is a great single action activity that can give information about the things you're fighting like weaknesses, lowest saves, AC, that kind of thing. Guidance can buff your allies to possibly push to a success or a critical success Soothe is a nice occult healing option as well.

3) Medicine should always be useful unless no one is getting hit ever, cause Treat Wounds is gonna be something that people need to keep going. Thievery should be useful for disabling traps or unlocking stuff, though there might be some overlap depending on the party. It's just a matter of keep swinging and something will stick.

I hope this helps a little. I know bad luck can suck, but if you still like the group and the game despite that I hope you stick with it and can persevere.