Biking is not always more efficient, as this quora answer points out: https://www.quora.com/Is-it-more-energy-efficient-to-walk-run-or-bike-up-a-hill/answer/Raj-Kumar-855
Regarding muscles, as an (former) cycling enthusiast, cycling uses a lot fewer muscles than running. The muscles used for cycling are some of the strongest in the body (hamstring, quadriceps). Compared to running which uses almost the whole body, including core muscles for stability, this is a downside for cycling, because it may not be as natural as running. But I wouldn’t say you end up tiring weaker muscles.
2. The speed at which balancing becomes effortless is roughly walking speed. It is actually difficult to ride that slowly (depending on the bike, load, windspeed etc).
Survey has been taken.