A: Merriam-Webster lists the symbol # for "hashtag." In definition, Merriam-Webster says it was first used in 2007 (Ref: https://www.merriam-webster.com/dictionary/hashtag#note-1) for use on social media. Also states there are many rumors and lists some of them, but that the origin of who coined it to "hashtag" is unknown.
If you want more history, start with octothorpe for #. The word just previous to "hashtag." Merriam-Webster dictionary gives definition as the same symbol: the symbol # (https://www.merriam-webster.com/dictionary/octothorpe) - who coined that as well is also unknown.
It may be prudent to read the "Did You Know" annotation under MW's "hashtag" and the "octothorpe" definitions. The origin of both names for this symbol "#" is stated as unknown. Rumors around it abound on the internet. The first known use, Merriam-Webster lists for hashtag is 2007. For octothorpe, 1971.
That said, I was born in 1958. I remember it as "octothorpe," and what a rotary telephone looked like before any # in the rotary dial. When "Ma Bell," or Bell Telephone Systems, AT&T had a country wide monopoly on America's telephone system. A system complete with manual handset. A 3 or 4 digit telephone number. Bigger cities had bigger numbers. A human operator used to connect many local, if out of your city to the next city over 5 miles away. All long distance calls were operator assisted. It took time dialing. I think back on all the prank calls I made. How long it took me to keep re-dialing to prank the person or store asking if they had Sir Raleigh (cigars) in a can, and if they did to please let him out. Voice disguised as everyone in town knew each other. Hang up in a time period before phone calls had capability of being traced.
I also remember this flier from 1969. Bell Telephone Systems revolutionary system of "One touch dialing." (see "Bell Telephone Laboratories - "New Experiment in Telephony" Advertisement Flyer. Oct.1969 http://www.obitoftheday.com/post/42841080179/johnkarlin). You can enlarge it and read it for yourself. This system introduced a phased in use, as old operator exchange of dialing "O" to get the operator person to manually to put your call through, you pressed what they coined as the # "octothorpe" key to get into Bell's new revolutionary, operator-less system. I cite myself as the reference, since I was there.
For "hashtag," you can read articles such as "The History of #—and 6 Other Symbols that Rule Twitter (and the Web.") (2013) Time. K. Houston. He got the "octothorpe" right. Many other tidbits, some are conjecture or the rumor mill on who coined "hashtag," but I defer to the caution of Merriam-Webster on this and the same listed under their definition for "octothorpe," that who is responsible for coining these two words is unknown, and leave the rest to you.