Kamae is not just basic it is essential to executing proper technique. Kamae or posture is not just about which foot is forward or not, it is about proper body alignment and maintaining balance not just at the beginning or ending or a movement but also during the movement. It is probably the hardest thing for a beginner to understand or learn to do. A good sensei spends a lot of time showing, testing and correcting your posture. You may be able to practice standing with one foot forward and arms extended but you need a partner to test if you are maintaining your alignment and balance throughout your movement.
I feel compelled to underline the fact that posture and Kamae is not - as far i see it - the same thing. Actually the first can exist without the second, Also it is not all Aikido senseis that put much emphasize on Kamae.
Also it is maybe necessary to point out that ukemi is - at least - two different things. The way we fall - and the way we give an attack for the person executing the technique. The first you can practice by yourself. The latter is not something I believe can be trained solo.
Just my thoughts on the matter