DI:
Away from lateral combos
Up to survive in air and sometimes ground
Down to survive on the ground.
When you attack, any directional input is ignored minus in the air when you press left or right, so whenever I attack, i'm always holding either up or down ALWAYS. Get used to holding those directions in the middle of everything as you approach. Always assume the risk of a hit. If you always hold down on the ground, you'll crouch cancel and stay on the stage probably until 100%, and then DI'ing up helps you survive and giving you the mobility and options of being off and high rather than off and low that comes with CCing. Always be holding a direction, you dont need fast reaction times for good DI, unless you're smash DI'ing certain attacks (which you should, like the shine, or when Sheik d throws you, DI away, and then smash DI up the slap). If a combo is started, you should know whats coming next and generally you DI away, such as falcon's or mario's uair combos.
Returning
For getting back vs marth and roy... there are 3 options really, and you should mix up the 3.
1) If you can, recover way way high so that as you enter stage space they cant jump and hit you, its no use going high and entering the stage space if they can still hit you. From there you can drift about and choose an appropriate time to FF when you think the opponent wont be able to react to your landing in time.
2) Go in from below and Up sweet spot, with an emphasis on the sweet spot. With a sweet spot, forward smashing becomes very difficult, and also, if the marth or roy has anything but perfect spacing, the umbrella will most likely hit (it has a surprising horizontal range if you ride the wall) and so now you have the ledge. More on the ledge later
3) Up B, and just go just barely in front of them to grab the ledge. a crude diagram:
if you Fast Fall and open the umbrella right before grabbing:
Y...P..|......./
O...A..\.....|...SWORD RANGE
U...T....\....\
R...H......*-,.*-
................¯¯¯LEDGE
OR... you can just slowly float back. This is an excellent feint as the opponent will most likely wait for the FF
Y...P.|...../
O...A.|...|...SWORD RANGE
U...T.|....\
R...H.*-,_.*-
.............¯¯¯LEDGE
And there you are.
From the Ledge
If you're above 100%, more often than not, ledge attack. Its weird speed and ******** range make it far better than the below 100% ledge attack. Try to never use the below 100% ledge attack. Other than that, a switch up between rolling, standing, and jumping (dont underestimate jumping, its hard expected from Peach and leads into an... aerial assault >_>). You can switch up and actually try ledge hopping nair every so often, which I have met with some success. another nice reentry is ledge hop wavedash d smash, which I have also been able to use effectively. (Note that the timing has to be pretty tight, you can wavedash pretty early after the DJ from the ledge, so do it ASAP) Another cute trick just to get your opponent to reevaluate his options while you try to get back is Peach's pseudo ledge stall. If you press back and immediately air dodge up, you'll air dodge straight up and just grab the ledge again. Now, if you fast fall at a certain time frame (a very small one, cause you can still FF through the ledge) You have about a frame or 2 only of vulnerability during this. Most opponents will react instinctively and attack you right after the air dodge, but you wont be vulnerable until right before you grab the ledge, giving you an opprotunity to get back on. I should note that the FF is hardly necessary, as the point isnt to stall with invincibility but to feint your opponent into thinking you're vulnerable at times when you are not. You can also simply change the angle of your air dodge slightly outward or inward. Outward to give yourself more space and once again get your edgeguarder to follow you, or inward just to get back onto the stage.
Remember, vs marth there is no sure fire way of getting back on safely, it really is all about mindgames, and these are all the tricks I know.
