Shoot away, box speed (or 1/3 to 1 stop below box speed, if that's your habit) and develop normally. Film that age (either 2018 or 2015) isn't old enough to worry about speed loss unless it's been badly stored.
Do, however, remember to let it warm to room temperature before opening the sealed package, either 35 mm film can or the plastic wrapper on 120.
No, that's not old and both will be fine. ISO 9002 compliance meant manufacturers had to reduce expiry times, it has more to do with stock rotation than anything else.
I don't develop my own film but send it out. But if I was going to test a new camera, I would use a fresh roll of film. With old film, if it doesn't come out exactly right, you're gonna wonder whether the film itself being old might have caused an issue or the camera.
It's always a good idea to deal with a constant which means fresh film when testing something. It's not like you have to discard the old film. Just save it fir later when you know the camera is ok and you know if you have to make any adjustments to exposure to offset camera issues.
No, and shoot box speed so that you are working with a known quantity. Shooting less than box speed will only yield information that the film transport system works, better to leave the camera on the shelf for another ten years.
Ditto that. I just shot a roll of Portra 160 NC that expired in 2008 (color, long since expired, mostly refrigerated). Shot at box speed and developed normally. Shots look good except for a slight green bias that was easily correctable in post. Films hold up better than you might expect.