Why is this test more reliable than other Enneagram Tests?

Basically, the enneagram is not a test, but a theory. An Enneagram test is meant to help you find your type on the theory. Many people miss the potential of the Enneagram because of all the low quality tests present on internet. So many people fall on a type which does not really correspond to them and find themselves directly disappointed, taking their result as such. Which is a shame because the Enneagram is a theory that has a lot to contribute.
That's why this Enneagram Test was originally created, precisely to be more efficient than the other Enneagram tests and to allow more people to discover its potential.
A good enneagram test requires, in addition to a good choice of questions, a serious post-analysis of the answers, to optimize its algorithm, and give more reliable results. It has been done at the launch of this test.
Moreover, this Test does not mislead the users when they have close results but it specifies it very explicitly and it suggests the different possibilities.

Why is the Enneagram more useful than the MBTI for personal development?

The MBTI is a test based on cognitive functions which is used to learn about how a person is used to perceive and process information. It is used by companies at recruitment.
The Enneagram is used to learn about a person from its heart by describing a compulsive circle around which he would potentially turn his entire life. While the MBTI rather brings elements on the "how" of a behavior, the Enneagram learns a lot more about the "why".
The Enneagram is used by psychiatrists to better understand their patients, as well as associations, some of which help prisoners to reintegrate in the society.

Can we have several Enneagram types?

Most people don't, and that's good. Because initially, an enneagram type is not a personality type, but a compulsive circle. Since trying to manage just one of them is not easy at all, imagine several... If this happens, it would probably be in an extended "unhealthy" stage, during which compulsions of other types can be added to the basic one.
On the other hand, anyone can have personality traits of several Enneagram types, and it is usually the case! We are all capable of developing all personality traits of all types, without being part of these types. Especially when we are "healthy", and we are more in "agreement" with ourselves.