Use of "Might" and "Might not"

The word "might" is used to express possibility in favor of something. Negating it expresses the opposite. Technically, "might" allows for the possibility of "might not" but it doesn't necessarily offer that as a possibility to be considered.

I might come to your party.

This gives the listener some hope that you will attend the party.

I might not come to your party.

This alerts the listener of the sad possibility that you will not attend.

I read "I might be coming..." to express the possibility but not necessarily the likelihood of attending, and "I might not be coming..." to express the opposite. If I wanted to express strong possibility, I would say "I will probably [not] be coming...". A lot depends on context; for example, "I might be coming, assuming I grow a second head" is not the same as "I might be coming, unless I get one of my infrequent migraines".

"Might be" means the lack of certainty, as you know but about the probability of happening I doubt if there is a exact number, when someone says "I might be coming" means the probability of coming is more than not coming and vice versa. but in both cases the speaker is not sure about what he/she is going to do.

Hope my answer is understandable!

"Might" and "might not" are vaguer than some other phrases about how likely the event is, and usually mean an intermediate amount of probability. Very soft bounds would be that "I might be coming" is in roughly the 10% to 50% range, though those are very inexact (and of course vary from speaker to speaker and situation to situation). (In particular, "I might be coming" can easily extend a bit above 50%, though as the event becomes more likely, people will start saying things like "I'm probably coming" instead.)

On your list of phrases, "I might be coming" is a bit less likely than "It is likely that I am coming", but more likely than "It is slightly possible that I might be coming".