Game Related
(I play Rosalina & Luma for what that's worth)
6) Any advice on how to gimp Diddy? (I know his recovery is supposed to be easy to gimp but I haven't found it to be.)
The Monkey Flip has absolutely no hitboxes/grabboxes if not forward Diddy, and if you expect your opponent to do it, you can react in many ways when you see te flash, including her USmash, and even then, the grabbox only covers diddy himself(think at Falcon's Raptor Boost), being vulnerable to each of her disjointed hitboxes, the kick has low range by himself, so a timed attack should take him out however, you'll always want to attack him at his blind spot.
The Up B is what makes me think that Diddy has the worst recovery in the game.
First off, know that uncharged, it'll travel less than the aerial version of Tornado Uppercut's distance, so if a Diddy is less distant than this anticipate them, otherwise it'll be easier to intercept his recovery. During his Up B, if hit either during the charging or during the fly, no matter if by Sonic's Spring or Warlock Punch, his Barrels will fly off for half an hour and, until they explode, Diddy cannot use them again, and usually Diddy Kong can't charge enough to come back if he gets hit under the main platform of Final Destination, but be prepared for another hit nonetheless. Remember to be decisive and unpredictable.
7) What's the best way to deal with Sheik's Bouncing Fish? Is there any way to punish Bouncing Fish on shield?
I think that you can close distance after the hit to gain stage control and take a chance of applying a reaction to the Sheik, but there isn't a true punish on shield. However, it suffers high landing lag. Finally, if used for recovery, you'll want to shield since recoverying with the Bouncing Fish usually has as first aim to take you away from the ledge.
10) Is there any way to escape Luigi's down-throw to Cyclone combo? Especially at death %?
I'm not entirely sure, but I think that that combo works only if you DI, while he can Nair/Shoryuken when you don't, but he do both relying on a read, so don't get read. Also Luigi hasin general shorter range than Rosalina (aside for her grab) so don't let that happen in the first place.
On a side note, the Cyclone has an autolink angle, being techable if he drsgs you from the air to the ground, akin to Rosalina's Fair, but Luigi's down B has much more ending lag compared to Rosalina's Fair landing lag so be prepared.
Hope that it helps.