Your trip is too short to go to Versailles, you will barely be able to cover the minimum in Paris, LOL
Skip it this time, its better in spring thru fall anyways so you can enjoy the exstensive grounds. It is easy to get there though, and only about 1/2 hr by train. You can get a train and admission combo ticket called the Forfait Loisirs, about 19 euros. Allows you to skip line too.
You will not need to make reservations for museums, I have never done so , and I go in major tourist season. Do plan your visits to certain Musuems to avoid most lines.
May I ask which museums you are interested in?? I do suggest you definately do the Louvre, especially if you don't do Versailles. It was a Royal Palace also, and if you visit Napoleons Apartments there, and see the Crown Jewels, it will give you a taste of the splendor the Royals lived in a la Versailles.
I like the Orsay also, and didn't find lines too bad even in summer, although I have heard others complain. I suggest you go first thing when it opens, actually just before it opens, to avoid waits.
At Louvre there is no need to wait in long line at all, just enter through shopping mall or metro entrance level and buy tickets from machine, quick and easy and I have done this many times now.
The Towers at Notre Dame have long lines . And the Musuem pass does not let you skip them, same with Eiffel Tower, you must just plan to be there before opening.
I don't think lines will be as bad as they are in winter as they are in summer, although I am sure you are not the only tourists planning Christmas in Paris, Have fun!