grammar - Proper use of the conjunction "whether"

I've come across several different ways of using the conjunction whether.

First, the terse version:

"Jon was deciding whether to go outside."

That sentence sounds incomplete to me, because it's missing the "or part" (implied by context). When I read it out loud, I think to myself that the sentence should have been written like this:

"Jon was deciding whether to go outside or not."

But I keep seeing it used in both ways. I've also seen it used like this (similar to the above):

"Jon was deciding whether to go outside, or stay inside."

Like this, in the middle of the sentence:

"Jon was deciding whether or not to go outside."

That sentence structure doesn't sound right to me.

And, lastly, like this:

"Jon was deciding whether to go outside, or if he should
stay inside."

This is the most comfortable to read to me, but it feels a bit too verbose.

Are all the above sentences grammatically correct, and do they all have the same meaning?

Answer

The standard usage is:

whether this or that

where this and that are both phrases/clauses that should be parallel.

The second clause is commonly elliptical if it's the negation of the first.

So:

whether he likes it or he does not like it.

becomes

whether he likes it or not.

Of the examples you gave, "Whether to go outside, or if he should stay inside" isn't considered standard because it is not parallel. It should be "whether to go outside or stay inside". Also, "Whether or not to go outside" isn't standard, it should be "whether to go outside or not".