The Guitar, Marty's transparency, Road and Doc's trip make no sense

So Marty starts disappearing, says he can't play. Fine.

However, when Marty's body starts becoming transparent, how does that not affect the guitar in any way? If Marty's body is becoming partially ethereal, what's holding up the guitar? Is Marty suddenly learning telekinetic powers?

Also, why is Marty disappearing so 'selectively', shouldn't it be more uniform, and why does everyone else in the photo disappear by body parts (head's gone, etc.), but Marty is allowed to 'fade out'?

What I mean is, why is his HAND becoming transparent against his face, but his face remains? Either the whole body should be transparent, so we see through Marty's face as well as his hand, or if it's this kind of 'layered' transparenty, we should see Marty's BRAIN because his face layer becomes transparent. Why only the hand? Also, why is it hand instead of the SKIN of the hand so we see the muscles, bones, etc.. which should ALSO be transparent, so we see the blood and ..you get the idea. This makes no sense! (Although it IS closer to the 'his head's gone'-type bodypartial disappearance, I guess)

Now, let's talk about the road. Doc says there's not enough road to get to 88, when the EXTREMELY LONG, STRAIGHT ROAD is clearly visible to us, and a sports car like DeLorean should easily get its speed to 88 (if it can do it in a parking lot, after all!).

That's not my point, however. My point is, Doc can CLEARLY get it up to 88 very, VERY quickly and easily on THAT VERY ROAD in the evening, and so quickly, it doesn't seem to require much road anyway, so how come the road is suddenly too short? This makes NO SENSE.

Related to this, remember how in the so-called sequel, they worry about 'thinking fourth-dimensionally' (never mentioned in this movie), and wonder if those indians will be there or whatnot. Doc says they won't even be there (without being able to even know, so he shouldn't say this), and then, haha, they are anyway (and do not get startled when seemingly DIVINE EVENT occurs with a loud bang and explosion).

Now, why isn't Doc worried if the road will still be there in 2015? What if there was construction in that area and they changed that road? These things happen in 30 years. How could Doc know and be so sure that he'd risk certain death from hitting some factory wall or materializing inside concrete in the future? He doesn't consider it at all, he just .. goes? What?

This guy is so meticulous he builds a huge model of the whole square just to explain his idea, and he doesn't even take two and a half seconds to ponder about the unknowns and the dangers of time travel?

You can never know what's going to be at some location after 30 years, but at least you could take precautions, like selecting some 'most optimal area' or 'least likely area to have a wall right next to where you materialize' or SOMETHING like that.

Doc just drops off Marty and GOES, without any concern? What kind of sense does this make? Heck, even Marty CRASHES INTO A WALL when he returns to a FAMILIAR PLACE IN HIS OWN TIME.

How can Doc know he won't?

Crashing into a wall at 88 mph is not exactly SAFE, you know. Even if you wear seatbelt and have airbags and start breaking with ABS immediately, it can still hurt quite a lot and injure you seriously to crash into ANYTHING. I have been in a simulator that only goes to less than 10 mph and then suddenly stops, and that seatbelt pull was so hard I ached for most of that day.

88mph?? The pain would be deadly or intolerable. But sure, let's just GO, without any kind of consideration.

I wish movies made sense, it'd be so much more enjoyable to watch them..