1: Repetition of "to"
The list is as follows:
not only "to win"
but also "break"

Since "to" is after not only, it cannot be considered to go with break as well. That is why we need to repeat "to".

You would be correct if the sentence were written as

TO not only "win"
but also "break".

Now to is considered to go with both win and break and hence you should not repeat it with break.

If you think better with parenthesis then you may think of it as follows:

to (not only win) (but also break) - To common
not only (to win) but also (to break) - To repeated

2: Verb-ing form
The list is as follows:
Neither a threat to me
Nor intimidating to anyone.
The list above is not parallel since it is not consistent.
threat is not parallel to intimidating
So to make this list parallel we will need to make both as nouns or both as verb-ing. Thus:

Neither threatening TO me
Not intimidating to anyone.

The use of "to" after threatening makes sure that it is not used as a verb.

You are right about the fact that "intimidating" is acting as a noun in this sentence so we have a noun as parallel to noun, so why do we need to change threat to verb-ing form. The reason is "consistency". We always need to make sure that our lists are as consistent as possible.

I hope this helps.

Thanks for the detailed response rsadana.

If I say I like to play, swim and dance. I do not need to repeat to with swim and dance, but if I say

I like playing, then I have to use swmimming and dancing. The point I am making here is that, to does not need to be repeated; however any other parallel parts of speech needs to be structurally parallel.

I did read somewhere in the MGMAT SC guide that a noun and a Gerund can be parallel, it also said something about a complex gerund, something I do not recall, thats whay I thought that a threat and intimidating can be parallel.

Also, if I am using threatening it seems to have changed the meaning from a perceived threat to a verbal threat. For example, I can write I am threatening you blah blah, which is a verbal threat. Or A tiger is a threat to deer's existence, here threat is a perceived threat.

I hope I am able to explain my question.