Is this itinerary possible?

Day 1: Arrive in Tokyo, visit Studio Ghilbi Museum, Imperial Palace
Day 2: to Lake Kawaguchi, visit the music box museum
Day 3: Disneyland
Day 4: Fly to Okinawa, visit the parks
Day 5: Fly to Osaka
Day 6: Universal Studios
Day 7: Explore Osaka then fly back

I have checked, the duration for the transportation seems okay but I am not sure if for instance the time we have to spend at the airport before our flight and all that. Help and advice would be much appreciated! :):)

Day 1: Arrive in Tokyo, visit Studio Ghilbi Museum, Imperial Palace

You have to arrive in the morning and have a reservation for the Ghibli Museum (for a feasible time slot) in order for this day to be possible.

Day 4: Fly to Okinawa, visit the parks

It is a 3-hour flight. You should leave central Tokyo about 90-120 minutes before departure. And you will require an additional 30-45 minutes to get into central Naha. Not sure what "parks" you intend to visit, but access to these parks may take additional time. The famous aquarium, for example, is an additional two hours from Naha (one way).

So, if you were to take a 6am flight out of Tokyo, you could make it to central Naha by just before 10am. And to the aquarium around noon.

Domestic flights in Japan are generally convenient. You do not need to check-in until 20 minutes before departure (some low-budget airlines require earlier check-in) and the time lost in security checks tends to be small. I recommend to arrive at the airport about one hour before flight departure just to be secure (mainly in case there are delays in getting to the airport).

Thank you so much for your help! I am intending to go at the end of the month to Okinawa to see the Cherry Blossoms! So probably will rent a vehicle to travel around.

Another enquiry though, should I get a hotel that is in Shinjuku or Odaiba? When I am in Tokyo, as I realised that Odaiba is nearer to the Haneba Airport but it is far from the places I want to visit on the first day. Thanks again:)

Shinjuku sounds central to the places in your list. Transportation options out of Odaiba are very limited.