Learn English – “They have lived here for 10 years” Vs “they have been living here for 10 years”

First of all, it's unusual to use the word could in this context- it would mean that it is possible that they have already arrived, but you don't know for certain. If that's what you meant, both sentences are correct, but the second is more idiomatic.

If you meant that you expected them to arrive before now, then should is the correct word to use.

It is ten o’clock. They should have arrived now.

This means that you expect that they have arrived, but you don't know for certain whether they have arrived.

It is ten o’clock. They should have arrived by now.

This means that you expected them to have arrived, but you know that they have not yet arrived- you are expressing concern that things are not going according to plan.

I lived here for two years.

Means you lived there in the past and do not anymore.

I had lived here for two years...

You are right that this could indicate you still live there. If you said "I had lived here for two years when my brother came to visit", you are talking about how long you had been there up until a fixed point in time. You may still live there, you may not, context would determine. Obviously, if you were sat inside your house saying "I lived here..." then you clearly still do, but you could use the same word "here" when pointing out somewhere that you used to live.

I have lived here for two years.

For reference, this is how you would state how long you have lived where you presently reside.