did they even watch godfather 1 while writing godfather 2?

Agreed.

The screenplay has Sonny's murder occuring in the Spring of '46. But in Jenny M. Jones' book you get a glimpse of FFC's Sicilian notes that state those scenes take place in 1948.

The whole timeline is so screwed up. If the '48 clues above punctuate the timeline, then we must suppose that after murdering Sollozzo, Michael was in exile in Sicily for most of '46, all of '47, and some undetermined part of '48 (The Republican convention was held in June). So, sonny is still alive at that time as he beats up Carlo. Thus, prior to Sonny's death in '48, Vito lay in his sick bed for all of '46, all of '47, and some undetermined part of '48; possibly 2.5 years. Seems implausible.

I don't think he lay in his sick bed for two years, but the attempt on his life had severely weakened his health -- he was shot five times. I think he was already in the process of turning over complete control to Sonny. When Sonny was killed, he needed to start that process all over with Michael.

Bonasera manipulated the year on the tombstone on purpose.
The undertaker's revenge for the humiliation back in 1945.

That makes more sense than anything else I've read.

but as i explained before the massacre occurring in 50 is impossible going by the timeline

Danito was joking.

Hyman Roth pressured the Senator to give false information about the timeline during the hearings. Roth was a 1950 loyalist which is why he was killed.

Well, there wasn't enough time, Michael. There just wasn't enough time.

Was Vito lamenting that 4 or 5 years wasn't enough time, or that 8 or 9 years wasn't enough time?

Therein probably lies the answer. But there's no answer beyond anyone's own conjecture. The timeline was the weakest part of both the book and the films.

But when they wrote this stuff over forty years ago, they weren't picturing a bunch of lunatics with no lives (like us) discussing it on the Internet all day (Or as FFC famously asked Puzo during filming in 1971: What's the Internet? ).

I always wondered what Vito meant by "wasn't enough time." The novel states that it wasn't enough time for Vito to "wire" his political contacts into Michael. But, so what? He was planning to move the family to Nevada and go "legitimate."

TB, of course the novel does not contain such a scene. I think FFC put it in there as a way of conveying to the audience Vito's reluctance to involve Michael in the business. I don't think it has anything to do with setting Michael up with the politicians. I think he just meant that there wasn't sufficient time to see Michael embark on a path to become "senator Corleone".