Really, why do you need to kill his grandfather when you only have to kill the mood with some annoying, badly-timed phone calls and interruptions. Smaller chance of going to prison, and it leaves out the whole murder thing -- although whether or not knowingly preventing a known conception from occurring is akin to the murder of a future individual should be left to the philosophers.

As for the movie trilogy timeline, time travel is one of those devices which can be handled in a thousand different ways. As long as a story is internally consistent. The argument ensues from the fact that many viewers don't think Back to the Future is actually consistent. The Rules are what the Rules Need To Be at whatever point the script calls for them to be that.

Last two notes: since my early days on social media, I have been seeing Photoshops of the DeLorean's dashboard clock saying "today is the day Marty McFly landed in the future" -- and in every instance, it has been wrong. Oddly, I haven't seen it posted recently.

Secondly, as of tomorrow, Back to the Future is a time travel movie that takes place totally in the Past. This is actually a cool sub-genre of movies, watching a past time portrayed as future events extrapolated from some either further point in time. Cheesy results, sometimes, but cool.