Generally your game-plan is to get DK into the air and keep covering his landing by sharking the stage when he's trying to come down, which Sonic should be excellent at because he's basically fast enough to cover wherever it is DK's trying to escape to as he's landing. okay ggs man
upsmash can cover a lot of DK's ledge options so try that more. Also you should be more patient when trying to back air DK offstage, you can hit him through his up-b if you time it right. Upsmash at kill percents that is, but don't just charge it at the ledge. Stand at a distance when you can punish getup attack or a roll-on but also be ready to react to a regular getup where you can run in and upsmash.
Nah if bair were disjointed he'd be way higher on the tier list, haha. There are 4 frames with no hitboxes between each hit of Samus' fair so I'd say my bair is sneaking past your first fair hitbox and hitting you in the 4 frames before the next hitbox comes out.