I have a very short commute (2km to work) and commuted most of the summer on my hard tail mountain bike until last week. My mountain bike was a 2008 Kona Fire Mountain with 26" wheels with good tires for on the road. Road fine and did the job, put a rack on it and worked well. This bike was also used for bike rides with the kids and pulling a 2 seater bike trailer. I ended up having the opportunity of buying a 2016 Kona Dew Deluxe with 700cc wheels and the ride is night and day. Full 1 minute faster (originally only a 6.5-7 minute commute) and much, much easier to maneuver with the skinnier tires and no front shock bobbing around.

My suggestions, get either a 1) hard tail mountain bike with appropriate tires for all of your uses and have a shock with front lockout, 2) get 2 bikes, one for commuting and one for other uses. This way you also won't miss out on commuting or other uses by having one bike down for repairs etc.

Good luck