[susan53]

Question 3

First of all, the sentences that you suggest are wrong here. This is the last time implies a future or on-going event. You could say :

This is the last time (that) I'm going to tell you.
This is the last time (that) I'll tell you.
This is the last time (that) I'm telling you.

Here "that" is not a relative pronoun (as your version suggested) but a subordinating conjunction joining the two clauses. And "that" as a subordinating conjunction can always be omitted and usually is in neutral to informal style. It's more likely to be included in formal style. Frequency again...

Other examples : He said (that) it was raining; It's important (that) you should go.