Amsterdam to Ghent

You don't need to have a car for this route. The fastest train connections will take about 2.5 hours using the Thalys train to Antwerp and changing to a regional train the rest of the way. If you left early enough in the morning this would be doable. You can find schedules on the Deutsche Bahn and you can buy these tickets either on Rail Europe or Thalys. I hope this helps some.

Yes the altarpiece is undergoing a thorough restoration, but they do it panel by panel. The alterpiece is still at her usual show place in the St Bavo’s Cathedral in Ghent, so still able to visit. You can see the restoration behind glass at the Museum of Fine Arts also in Ghent. Two other museums have also a theme exhibition about the alterpiece, look for that at the website of the museum: www.mskgent.be ,select English there.
In the cathedral there is also a copy of the alterpiece, actually at her original place in the Vijd Chapel. If you have a ticket for the original you can have there an explanation of the painting, as supposed that the first has to be admired in silence.
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Is the focus on visiting cities, the train is the quickest and most convenient way to move around in The Netherlands and Belgium. A car is usefull if you want to drive through the country-side or want to visit remote places who are too time consuming to visit by public transport. Best way from Amsterdam to Ghent by train is highspeed to Antwerp and intercity train for the final leg. In Ghent take tram line 1 just outside the main "St. Pieters" station, direction Evergem or Wondelgem and exit at Korenmarkt for a short walk to the cathedral. You can use the same line for the visit to the Museum of Fine Arts, what is on the way back to the station (direction Flanders Expo), some walking is needed.
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Nevertheless thinking about a car, please ask questions.

The only advice I would give about the Adoration of the Mystic Lamb is "The earlier the better". First time in Gent, I was there just after 9 in the morning, and I walked right in. By 12, when I walked by St. Bav's again, there was a line out the door. Every time I've visited Gent subsequently, same thing.

I was yesterday in the Museum of Fine Arts (MSK museum) to see the restoration of the alterpiece and almost all the back panels are there now, so to see everything you have to go to besides the cathedral also go to the museum. I think all the frontpanels, being the best part of the alterpiece are still in the cathedral, but nevertheless ask at the ticketbooth in the cathedral for up-to-date information as things can change during the course of the restoration.