Largely what you said is not true. On evenings when the Christmas party takes place the park closes to the public at 7:00 pm, not 6:00 pm. Guests attending the party may enter Magic Kingdom starting at 4:00 pm.
From my experience Magic Kingdom crowds during the day are exactly what you'd expect based on the day of the week. The early closing hour DOES NOT influence crowd patterns! Sundays are very busy. Tuesdays and Thursdays, not so much. The challenging days are Fridays and Sundays because typically these parties are well attended (sold out), so there is a massive crowd of people in the park between 4:00 and 7:00, the three hour overlap of public and party guests.
Crowds in the other parks are generally what you'd expect based on the day of the week. I never notice a significant increase in guests in the other parks due to the Christmas parties. If anything up to 20,000 attending the Christmas party remove bodies from the other parks!
The typical tourist is in Orlando for 3-5 days, not a week or two.
The single biggest impact to crowd patterns is Magic Hours. If you're not taking part or eligible for Magic Hours then avoid the park that is offering them. If Magic Kingdom opens early to Resort guests then go to another park. If EPCOT is staying open three additional hours then avoid that park in the evening and go in the morning instead.