Great guide. It was pretty informative. I knew all of the basic move properties and when to use them, as well as Din's Fire, but I'm still on my way to becoming proficient with Farore's Wind and Nayru's Love.
I have a few pointers against Ness (and to some extent, Lucas), as one of my friends is tMoC (a good Ness player from Melee and still plays him in Brawl), who I've played against several times as Zelda.
Ness and Lucas are both absorb happy against Zelda. One thing you can do with Din's Fire is have it explode
just out of range for them. I often explode them prematurely in order to stop their approach, or to get them to absorb. This also works in getting them used to the idea that you don't want to hit them with it, so you are more likely to land a Din's Fire when you want to. I've had a considerable amount of success against Ness with Zelda, though the battle is certainly not easy.
One thing you MUST be careful against both Ness and Lucas is their PK Thunder. Good players know how to hit you with the tail end, allowing them to score multiple hits with a single PK Thunder. On platform stages, this becomes a larger problem because you cannot air dodge through platforms at certain distances (making you vulnerable prematurely). The best way that I've found to deal with their PK Thunder spam is to use Nayru's Love. However, both Ness and Lucas are released from their concentration animation the moment you reflect it, allowing them to perform another one or score hits on you if you're too close. When extremely close to Ness/Lucas when they're using PK Thunder and you're airborne, the best solution is to air dodge (unless you get a clear shot to reflect the PK Thunder directly back at them). Even if this pops you up and allows for one or two more PK Thunders to strike you, it's often better than letting them start close range combos.
Also, you must be careful about using aerial attacks against Ness. His f-air is disjointed, so it will beat out all of your moves. Plus, if you attack him while he's shielding (even if you land behind him), he can SH n-air, f-air, or b-air which can easily result in losing a stock. I find the best way to beat him is by baiting him into attacking and then scoring hits on him. Keep your Din's Fire just out of reach of hitting him more often than scoring hits. Healing him is probably going to happen, but if you use Din's right, you'll rack up a lot more damage than you heal him. Always try to get him into the air, as he can't punish you as well when he's not grounded (and it allows for up-smashes, which are a primary source of damage and KO set ups).
You can also Nayru's Love after getting hit by PK Fire to turn the pillar of flame into your own projectile, causing an approaching Ness to get hit by it and set up into a KO move or juggle. Don't over use this as they will learn to expect it and hit you with a PK Thunder or max range dash attack, both of which set up into juggles or KOs.
On a separate note, I have a small amount of experience playing against a few tournament MKs as Zelda (in friendlies, though).
First off, MK's d-smash is considerably faster than Zelda's in both start up and lag. If you ever try to beat his d-smash, you will get hit (if he tries to beat yours, though, you have the possibility of beating him out).
Using Din's Fire like crazy is really the only option I've found against him. It often forces him into the air, where you can dash to up-smash (I hate hyphen smash as a term), or dash attack (while they're airborne) them relatively safely. Also, since MK's up-air has pretty short range, it is possible to time sweet spot d-airs when you are above him (obviously use discretion, since if you are high up and trade hits, you're toast). Up-smash goes through his glide attacks if you throw it out just a tad bit early. Fortunately, a lot of his approach moves will knock you very far away at higher %. You can use his own approaches to space yourself from him as a last resort when you can't seem to shake him any other way (this works on larger stages like FD).
I find that using f-air and b-air sparingly in this match makes it a
lot more effective. The less you take to air combat (aside from up-airs), they more risky the MK will become in their own aerial attacks (since they don't have to time to beat out your attacks). I usually end up using f-airs and b-airs only around 2-4 times per match in total (both of them added up), which is considerably less than just about every other move I use, but when I do throw them out, they often score me a KO or at least land with a sweet spot.
I am by no means a great Zelda player. You can watch some matches of mine against a MK player on youtube (links below). They're over WFC, though, so while nothing on them is definitive, it does give ideas to anyone who is curious about how to deal with MK. The MK is fairly decent, but probably not going to win any tournaments. Still, I think they are pretty good examples of what to do and what not to do (I get punished a fair bit in some of them).
http://youtube.com/watch?v=QAlxPS_rI3o
http://youtube.com/watch?v=4-6-SRpf7e0
http://youtube.com/watch?v=7yHYsk0y4ew
http://youtube.com/watch?v=8BbkS9u_hzY