了 has nothing to do with pastness. Don’t confuse it with the past tense or associate it with past-ness. It is instead a marker of completion of an action OR a change in state. 我昨天上学了 我已经吃饭了 These are completed actions. Like most completed actions they occur in the past, but the 了 isn’t a tense marker. The sentence 医生说她不该喝那么多酒 also took place in the past because but doesn’t get a 了 since it isn’t completed. The doctor said she shouldn’t drink so much alcohol, but the doctor has not changed their mind. They still think she shouldn’t drink so much alcohol, so the action of speaking isn’t “completed”, and therefore doesn’t need a 了 even though it’s in the past. The sentence 我要杀了他 isn’t even in the past, it’s an expression of desire and intent not a description of real events, but it DOES take a 了 to emphasize that you want the action completed. I can’t give you one rule that will describe every usage (I haven’t described 了 as in describing a change of plans or a change in state or as its used idiomatically) but seriously, it isn’t a tense marker and has nothing to do with pastness. Look for changes and completion and not time or what tense something would be in English.

While I think it is acceptable to omit 了 in sequences sometimes, you can avoid repition in other ways too. Compare: 昨天我去大学了。然后回来了我家。然后我吃了饭。 昨天，上大学之后，我回家吃饭了。 The meaning is a little different: what you wrote is a list of events in a sequence, what I wrote is describing the relationship between the events. It’s like the difference between “I went to school. Then I went home. Then I ate.” And “After school, I went home and ate/went home to eat”

You should never be upset because even the linguists are still debating 了 at this moment. 昨天我去大学 and 昨天我去大学了 are both correct in different context. When you just refer to a time in which you encountered an event, then 了 is not needed。Specifically, when you can use 当( I went to the university)时, 了 is not needed. However, when you want to report an event, 了 should not be omitted. Because 了 marks a point here. 我醒了-> you are not awake before that moment; 我到了-> you do not arrive yet before that moment. 我过了-> you do not pass before that moment. So when you say 我昨天去了大学了, it sounds like you had not been made it to class for a while before yesterday. And you report your going to the listener. 昨天我去大学（了）。然后回来（了）我家。然后我吃（了）饭。然后我去商店买书（了） In this case, you are just reporting a temporal order in which you've done a couple of things. We often say 昨天我去完学校后回家，吃完饭后我去商店买书了/or 买了书。In this discourse, the important event is that you bought a book, and other things are merely time reference. Thus, you can put 了 to strength it. However, it is right to say 昨天我去了学校，然后回了家，然后我吃了饭，然后又去商店买了书。In this context, 了 are marked after all the verb? why is that? Because you are counting event in an order. They all look alike to you. In my opinion, it is the same as English presenting time order: I waked up, brushed my teeth, and went out. Nevertheless, it is still debated.

I've been learning Mandarin for almost 4 years and I still don't entirely understand this lol. It's so damn complicated.