Ace backtracked to basically the start of the grand line to hunt for BB. If Sabo, who left before Ace, kept going, he would have gotten to the new world and had no real need to backtrack. Ace was made an effort to meet up with Luffy, but only because he was in the general vicinity anyway.
Of course, Ace also gained fame on Whitebeard's crew, so Sabo not meeting up with him after that is a little odd, but there's thousands of reasonable and plausible explanations for why he wouldn't, being part of the revolutionaries being an incredibly fitting one.
As for why he didn't attend the execution? May have simply been too far away. Luffy only made it with a day to spare through the love of a shichubukai, access to a special government current, and sheer fluke of timing and access. If Sabo was any further away, (like… the new world) well, making it in time would be, not only incredibly difficult, but possibly... impossible. Even if he was trying his hardest to get there.
Also, I prefer Ace's mispelled tattoo be an artist's mistake, that was far more hilarious and character building and said something about Ace's attitude. A symbolic gesture? Meh. But between the flag and the letter, its definatley being driven home, so thats probably what it is.
Considering Nojiko and Nami already had that sort of sybolism with tattoos, I don't care for it, even if its where it seems to be going.
@Deicide:
Yes. The destroyed ship
Like the one Igaram was in that Robin blew up? Or like Going Merry when it was smashed upon the rocks?
and floating hat are more than enough to send the message.
Like when Luffy fought Crocadile? Or when Ace fought Blackbeard? Or while Jimbei was carrying Luffy away from Akainu?
Its symbolism, sure. And its suggestive of death. But its Oda-cheat style sybolism.
You don't have to show the body to the readers for a character to see it with their own eyes. Also. Silhouettes are useful for that sort of thing.