1) No
They aren't 20" tires. They're 20" wheels. The overall diameter of the tire is the same - it just has a smaller sidewall than a comparable tire for the 18" wheel.
2) Only a little.
A tire is part of your total suspension system. There's "give" to the sidewall in a tire. A tire with a smaller sidewall will have less give than one with a bigger sidewall. Therefore, a QX60 with 20" wheels will ride very marginally firmer than 18". You'll barely notice the difference.
They aren't 20" tires. They're 20" wheels. The overall diameter of the tire is the same - it just has a smaller sidewall than a comparable tire for the 18" wheel.
2) Only a little.
A tire is part of your total suspension system. There's "give" to the sidewall in a tire. A tire with a smaller sidewall will have less give than one with a bigger sidewall. Therefore, a QX60 with 20" wheels will ride very marginally firmer than 18". You'll barely notice the difference.