There is an old-yogic piece of wisdom: to excel at a particular yoga posture (like Butterfly), do that posture! If you want to be able to sit on the floor comfortably, start sitting on the floor all the time. There is a Newsletter article I wrote a while back that addresses most of your questions: it is called The Sit Rise Test
. Read it. It is good.
Basically, you need to answer the question - what is stopping me? Why can’t you sit comfortably on the floor. The answer will be either tension in your muscles and fascia or tightness or compression in your joints, or a combination of both. Yes, you may be inflexible - but why? Knowing the answer will help you know what to do about it. But let’s assume it is tightness in the joints: specifically the hips, and to a lesser degree, the lower spine. Sitting on the floor when you read, when you watch TV, when your talk on the floor will open and strengthen your hips and lower back. It you are on the computer a lot, put it on your coffee table and get down there. Eat at your coffee table. While you are on the floor, change positions every 3 ~ 4 minutes: mix it up. Do Butterfly, Half-Butterfly, Caterpillar, Straddle, Sphinx … work towards Shoelace and Square Pose over time. [You can look up all these postures, including some videos of them, here
It won't take too long: after a few months of living on the floor you will be amazed at the difference. (But please remember: you can do too much of anything - if at any time this becomes painful, then back off. Start slow and work towards your goal.)
I hope this helps!