Tell me please if the following sentence is gramatically correct.
John, would not it be better to visit Rachel tommorow, if it would not, then could you please explain why?
What I am in doubt about is the use of would after if.
There is no problem using would after if.
But the construction would not it is most unusual. In fact, a quick Ngram check found no usages of it all. The usual construction is would it not.
That aside, you need either a semi-colon or a full-stop after tomorrow to break up your sentence.
John, would it not be better to visit Rachel tommorow? If it would not (be better), then could you please explain why?
Note that the use of be better in the second sentence is understood.
Alternatively you could simplify the sentence and omit the words it would and then altogether in the second sentence.
John, would it not be better to visit Rachel tommorow; if not, could you please explain why?
This is the form that most people would be likely to use. But you will often hear people saying things like:
If you would be kind enough.....
If you would consider going...
And there's no problem with this construction.