Marty's letter makes no sense

So, Marty painstakingly writes this letter - perfectly lettered with beautiful penmanship with a 1955 ink pen. That alone makes no sense - a 1985 teen with no training whatsoever, just happens to be able to accomplish this beautiful penmanship feat, where Charlie Brown never even learned to write simple letters without smudging, and he was a 1950s' kid! Not to mention how odd the ink pen is - it looks like one of those that have an ink cartridge inbuilt that you can refill, but then Marty clearly has a bottle of ink on the table, as if it's one of those quill-type pens that need to be constantly dipped in ink.

However, what makes even less sense about this whole thing, is that Marty ABSOLUTELY doesn't have to write that letter, and it doesn't matter if Doc tears it up.

Did everyone forget, he has a TIME MACHINE around him whenever he wants. He can just go back in time, since he knows where the plutonium is, he knows how to load it in and so on. All he has to do is use this knowledge, then he has as many tries as he has plutonium pellets, to try to save the Doc.

It doesn't matter that Doc is dead in 1985 (I mean, suppose Marty returned to the original timeline somehow), because Marty can just load a pellet in the time machine, go back two weeks to tell Doc all that happened and to convince him to abandon this stupid time machine idea or at least do it in a safer place, etc.

Heck, Marty could find out the exact time and date of the whole plutonium theft and simply stop Doc from doing it, put a Nikola Tesla book in his hands and the rest would be history (no pun intended).

Also, isn't it interesting, that just like pyramid schemers, Doc never goes FAR ENOUGH with his explanation of what would happen? He stops at explaining how Marty would be next to disappear, but NOT what would happen AFTER Marty disappeared.

Marty disappears because their parents won't meet - this would leads to there not BEING any Marty to go back in time to stop the parents from meeting, so their parents would meet, and there'd be a Marty.

Marty stopping the parents from meeting so Marty won't be born would lead to there not being a Marty to stop them from meeting so Marty will be born - exactly the same paradox would happen, as with 'The Terminator (1984)', if Skynet succeeded. I mean, with success, no need to send a terminator back in time would mean no success, and so on.

The more you think about this movie, the more you find things that absolutely make no sense (The same, exact family living in the same, exact house and Dave and Marty's sister living with their parents although now they're 'successful' makes no sense, for one thing).