With the chain grabbing I'm not so sure. My main character is Marth and you can only sort of chain grab with him.
The key to event match 51 is, (like you said) using Jigglypuff. To have a really good chance of beating it, try not to die at all until you have killed Giga Bowser. (If you use Jigglypuff's B right move followed by a rest on Giga Bowser you'll kill him nearly every time.) After you kill Giga Bowser all three times you just need to avoid Ganondorf and Mewtwo until you get a good item or just use Jiggly's charge B all the time.
For Yoshi in home run contest, all you have to do is short hop (a smaller jump which you get by tapping the jump button quicker) and instantly going into his A and down move. If you are fast enough with the attacking, you can hit sandbag over 2 000 feet. I'll make a video and show you. However, I'm not a pro at homerun contest and can't do any of the really crazy stuff yet which can get you distances over 5 000 feet.
If you haven't seen the Melee videos below yet, I strongly recommend you watching them because they teach you many advanced techniques which will make you twice as good as a Smash fighter. (Trust me they're great!)
Advanced How to Play Part 1
Advanced How to Play Part 2
Advanced How to Play Part 3
If you want to watch these you better have a lot of time on your hands because you need to watch them all at once for them to make sense.
Genesis 3:15-17 "And God said,'Let man be created in my image and have power over everything on earth. Brawl, Mario Kart, and even Legend of Zelda are yours to master and control..."