Learn English – At some point “in future” or “in the future”

Addressing your specific questions:

'Can you imagine how dirty it gets over time?'

When people talk informally, they are often a little sloppy. The present tense would be the perfect choice to state a generalization or natural law such as:

'White gets dirty over time.'

This idea was close enough for the speaker. Someone who likes to be more precise might say, instead, "Can you imagine how dirty it would get over time [if we bought the white pillow]?" But you might not enjoy living with someone who uses English precisely all the time. Such people tend to be nitpicky.

'The grease from your fingers will burn into the bulb and then it breaks.'

Here, two ideas have been spliced together into one sentence.

First idea: "[If you touch the bulb with your fingers] the grease from your fingers will burn into the bulb."

Second idea: "When that happens, i.e. when you touch the bulb with your fingers, the bulb breaks."

This is another example of the simple present being used for a generalization or natural law. And again, in a situation of informal speech you caught the speaker being a little sloppy.

'I hope it goes away overnight.'

I don't know how the English grammar experts would view this (if you want to know, you could ask over at ELU SE), but I will share how I see this example. My other primary language is Spanish, which has a subjunctive. English has one too, but people don't think about it much. In Spanish you really can't get away without thinking about it. In Spanish, this would be

Espero que se quite para mañana. ("se quite" is conjugated in the subjunctive)

I see the English sentence the same way -- I see goes away as the present subjunctive, which happens to be conjugated the same as the simple present.

Your questions were good, and you've done some careful listening and recording of what you've heard.

I would be remiss if I didn't point out a small but important misuse of the simple present tense in your question:

You wrote: I moved from Germany to California and since I'm here I hear people use the simple present....

Expressions beginning with "since" are notoriously easy to get tripped up on.

Better: I moved from Germany to California and since I've been here I've heard people use the simple present....

Or: I'm from Germany. Here in California I hear people use the simple present....

"Could" is a multi-purpose word. Sometimes it indicates potential:

He's a swimming prodigy. He could be the next Michael Phelps

Sometimes opportunity:

We could have dinner at that new restaurant which just opened over on 5th street.

And sometimes simple past ability:

There was too much traffic, so I could not get to the post office today.

In your examples you're mixing together potential and ability. If you want to focus on a possible future action, "could" is fine:

Could the plane take off before the bad weather rolls in?

But not if we're talking about the ability to perform some action:

The plane can't land on that runway. It could have landed on the one a hundred miles back, but now there's not enough fuel to go back.

Of course, sometimes there's almost no distinction between "potential" and "ability". Your train schedule is a good example. It's fine for me to ask:

Could there be another train?

because this refers to the possibility of more trains. But how about if I ask:

Could I take the next train?

Am I asking if it is possible for me to take the next train? Or am I asking if I am able to take the next train? Is there really a difference between these? If there are no more trains, you could answer either way:

No you can't (you're not able). The last train left ten minutes ago.

No, you couldn't (it's not possible). The last train left ten minutes ago.

To be clear, when specifically talking about ability, "could" only refers to past ability:

He can play the trombone.
He could play the trombone when he was in grade school, but now he can't.

As with anything else in English, there may be exceptions.