The problem with switching to playing defensively is that they're probably better at defensive/counter play than you are because that's their dominant playstyle. I've also had a lot of trouble with more defensive players, but I've had some success with waiting out an opening (like xman said, missed tech skill, poorly spaced bait, if you successfully bait a defensive move, etc.) and then going in and staying in. If your mobility's good enough, you can stay so close to them that they can't run away. Don't constantly try to hit them, just make it so that they can't safely run away to reset the situation; they have to go for a reversal if they want you to back off. Now it's your game. In this close-in game, Falco's shine is super strong and his slow dash is less relevant because his WD is good enough to keep you mobile.
It's kinda what magister54 was getting at with switching between modes of play. You play defensively and safe until they do something that makes it so you can get in close to them, but then switch to your normal super aggro style.
I know it sounds like this:
https://www.youtube.com/watch?v=ZGlQ7_OVIiI but you don't have to hit them, you just have to get close to them. Therefore, the definition of an opportunity is hugely expanded.
Crossups are really important for this style, as well as turnaround SHL. They both keep you close and keep the pressure on while still changing spacing drastically. It's dangerous because you're always in range to get hit or grabbed, so sometimes you have to back out in order to not get killed. This usually happens when you both recover from movelag at about he same time and they're outside shine range. I like to dash back turnaround SHL at this point or just wd back and reset. It sucks but it's better than losing a stock or getting chunked.
Also, this is really, really hard against Fox because he's faster than you. Now you have to use your longer legs to your advantage. It's doable but really hard and really dangerous.
If they're good at perfect shielding, SHL won't do much good against a DD. I guess you could shoot a laser low and then sh over the low laser after they reflect it, but that's hard and if they're DDing and they power shield, the laser will come at you at different times depending on when in their DD they reflected the laser. I know a lot of Marths and Shieks will only try to powershield during their dashdances because their wd, dd, and crouches are so low that a low of lasers will go over their head if they miss the powershield.
Falco's full jump gets you to the top platform probably faster than they can, so you can cut them off from the top platform by getting there first. Maybe hold shield facing away from them so you can shield drop bair if they try to hit you with something. Also, Falco's grab range kinda sucks, so you can't count on shield grabbing to get out of shield pressure.
vs shield, shinegrab op. vs swatting you away, I agree with magister54; sh forward waveland back is super strong. vs dd, never get into a rhythm because that makes it easier to powershield. If melee's a song, think of it like synchopation or jazz; you keep the song moving fast, but offbeats are emphasized to shake up what's expected. In other words, DD and laser at the same time.
I'll record what I mean and post it here if there's interest.