It’s well known that introductory phrases should be followed by commas. Here is an example:
To stay in shape for competition, athletes must exercise every day.
Consider now the following two sentences:
In the best case, it is like this. In the worst case, it is like that.
Suppose one wishes to write them in one sentence using the coordinator and. What is the right way to punctuate this new sentence?
In the best case, it is like this, and in the worst case, it is like that.
In the best case, it is like this, and, in the worst case, it is like that.
Another example that I often stumble upon is the following:
It is like this, and therefore, it is like that.
It is like this, and, therefore, it is like that.
What is preferable: sentences 1 and 3 or sentences 2 and 4? Is there some rule in this regard?