Personally, what sounds best to my ear is to make the verb past tense when using "made sure", and present tense in the other circumstances:
They made sure that we sat together.
Please make sure that we sit together.
They will make sure that we sit together.
I don't think your idea can exist.
In your sentence...
I can have had been reading a book if I could send a letter back in time to tell myself.
... the conditional if and past-tense makes your opening can illogical.
Instead,
I could have been reading a book if I could have sent a letter back in time to tell myself to read it.
-or-
I could have read a book if I could have sent a letter back in time to tell myself to read it.
-or-
I could be reading a book if I could have sent a letter back in time to tell myself to be reading it.
If you're writing fiction, and the idea you're trying to communicate is that you could be doing something else right now if you could change the timeline, I would say it like this:
If I can send this letter back in time to myself, then I can have read the book before it's too late.
When you try to say "I can have had been" you're creating a time paradox. Consider the following:
I can be reading if I can send myself a letter back in time to tell me to be reading.
If the above is true, why aren't you reading? And if you were reading, you couldn't be sending yourself a letter to tell yourself to read, so you wouldn't be reading.
So, like I said, I don't think the idea (as you've expressed it) is able to exist, the grammatical problem merely uncovers the time paradox.
Best Answer
No. The sentence you have created expresses the past tense. "He must have been coming" means that you assume he was coming at some time in the past, but the action is not completed.
What you can say is "he must be coming" or "he must be on his way". The word "must" used in this sense is not as absolute as it may seem. It simply implies that you are assuming something is true.