First time in Paris - decent questions regarding transportation and tourist attractions [closed]

About my journey:

I will be travelling to Paris in the beginning of March with my girlfriend and staying for 3 days.

We are landing at Paris Beauvais Tille Airport. I have unfortunately found that the airport is kinda far away from the city, but understood from their website that there are a lot of methods to get between the airport and Porte Maillot.

We will be staying at a hotel near Parc Suzanne Lenglen (Bd. Gambetta).

Questions:

1. Travelling : Hope that I am not wrong, but I was considering travelling through town by Metro (from downtown to the hotel, from a tourist attraction to another, etc). I searched for ticket prices on their website and found that 10 tickets are €12.70. Are the tickets available on any Metro line? (I will need to switch metros every now and then). Do you suggest buying a 3 day pass? Or better individual tickets?
2. Must see tourist attractions : I've seen where La tour Eiffel, La Louvre or La cathédrale Notre-Dame are situated. I would like to know what other must-see places you suggest (preferably somewhere downtown, but this is not a must as long as there is a metro station nearby). Any suggestion is welcomed - from museums to nice parks or picturesque buildings, monuments, etc.
3. Travelling between tourist attractions : Do you recommend walking every now and then between the tourist attractions? For example I saw there are 3.5 km between La Tour Eiffel and La Louvre. Is it worthwhile walking this distance? Meaning that if it is a nice area to travel by foot.
4. Admission fees for the attractions : I saw for example on the Louvre's website that Admission to the Louvre and the Musée Eugène Delacroix is free for the following visitor categories: 18-25 year-old residents of the European Economic Area (EU, Iceland, Norway, and Liechtenstein). Can anyone confirm? Are there any other discounts for students? Do they apply if I am a student in my own country and provide proper documents? I hope we will not be spending too much on entrance fees where we go.

Any other comments, proposals and any kind of feedback is more than welcomed. If anything else pops up in my mind I will put it here.

Thanks in advance !