Are the people who answer questions regularly on yahoo actually highly experienced professionals in their fields?

Not many, but there are some who post in particular subcategories and are quite knowledgeable about that topic (e.g., Mathematics or Computers). The vast majority of users here are just regular people with varied degrees of knowledge/experience in the topics they reply to, and some have none at all (and their responses often reflect that). 

I'd say ninety eight point five percent of us, aren't actually even experienced professionals. You'll find a few in a few select categories who are. Real estate and insurance are two more that you may find some actual professionals also in some of the Business and Fiance categories, Education, Home and Garden, and MAYBE one or two in Travel. Otherwise in many categories you'll find some who are actually knowledgeable, and others who as stated actually know nothing of the topic itself, are dreadfully misinformed, or worse.

Some are, especially in the serious categories. For example, in Chemistry, several PhDs will see your post.

Don't have to be.  Just have to have experienced the same difficulty or situation and solved it. Just spreading the knowledge.

I worked for years as a teacher, then in IT, in a woman's  shelter, managed PR for a new homeless shelter, and ran a successful online cancer support group. I speak four languages and have lived in four countries.  I am now slightly disabled so I have some time.
I picked up some useful information on the way.

One would hope so, however since this site is internet based, information can easily 
be copied and pasted as an answer. Others are point-gaming, probably cheating by 
having more than one account, switching accounts to give themselves a best answer.

Some of us are, many aren't.

Some are and some are not.

Heavens no. I'd say at most 1/3 of the answers are legitimate ones. The rest are just ordinary people venting or letting off steam...especially in the belief-based subjects like religion and politics.

To test whether the answer is legitimate, look to see if the answerer provided any legitimate sources to validate they answer. If there is a legitimate source, the answerer has at least done some research on the subject.

Some of them are but not all of them