The manga 9981 was not adapted from a novel but from a Japanese manga of the same name. The manga was first released in 1996 by the manga artist Nobuyuki Fukumoto. Although comics and novels have the same name, they are two different art forms. The storyline and character settings of comics are different from novels.
It depends on the specific comic and the source material. Sometimes it's a direct and faithful adaptation, making it canon. But in other cases, there could be significant changes that might make it not strictly canon.
No, Popeye wasn't the first. Before Popeye, there were other comics that were adapted into different forms. The history of comic adaptations goes way back and Popeye was just one among many.
The Titans comic adaptation has its ups and downs in terms of accuracy. Some characters and plotlines are closely followed, while others have been tweaked to fit the visual or narrative demands of the adaptation. It's a mixed bag.
The Sandman adaptation is mostly true to the comic. It manages to replicate the atmosphere and character developments well. Some storylines might be condensed or expanded slightly, but it still retains the essence of the original comic.
The 'Invincible' adaptation is mostly accurate to the comic. It captures the essence and major events, although there could be a few tweaks here and there to make it more suitable for the screen.