Learn English – I am neither robber nor police. Neither I am robber nor police

It is more usual to keep the normal word order for the first clause and use inverted order for the second clause of such sentences, in a not-neither or not-nor pattern:

She is not thin, nor is she fat.

She is not thin; neither is she fat.

(Of course, the most usual phrasing would be "she is neither thin nor fat", but I've assumed you're interested in inverted constructions in particular.)

Of your original options, I think 2 is technically correct, but belongs to a poetic style. 1 is incorrect (it might be technically correct if you changed "she is" to "is she", but would sound a bit odd). I would advise against such constructions.

In informal speech, people might say something like 1 with the "she is" word order and an added pause. It would then be acceptable very informally, but it wouldn't be an example of inversion. It would be a colloquialism (perhaps representing the addition of an afterthought in disconnected or disfluent speech), along the lines of "A good man, he is" or "A sunny day, it is".

The two examples you provided for "sometimes" are both OK and they mean exactly the same thing.

The case for "however" It would be very useful for you to provide a context of using "However". That way, the answer can be more helpful to you.

Usually I use "however" according to the following pattern:

statement1 however statement2

They can be in the same sentence (with "however" in the middle), or in consecutive sentences (with "however" at the beginning of the second sentence).

The meaning of the construct is: statement1 is correct, but statement2 brings a new point of view which is not entirely compatible with statement1.

Examples:

I like to work; however, I like even more to relax.

or:

Jane is very experienced. However, we cannot hire her because we require a different skill set.