Can I purchase a Major Improvement card that is underneath another Major Improvement without first having to buy the top improvement?
No, you have to buy the Major improvement that is above first. This isn't explicitly defined, but it is implied on p.2 of the rules:
An advantage of placing 12 of the 14 new Major Improvements underneath others at the start of the game is that the use of the cards does not need to be explained in detail at the start of the game. The only two new Major Improvements that need to be explained are the Peat-charcoal Kiln and the Forester's Lodge.
There are also Minor Improvements which change the order of the stacking, again implying that the order is significant. For example, the Pastoral Letter (p.30):
If the Well has not yet been built, you can immediately place it under the Village Church. (The Well and the Village Church swap places.)...
Major Improvements are now stacked. There are 12 stacks of 2 cards each. Only after the top card is bought, the other card is revealed and available for purchase.