This one is kinda difficult to quantify so I'm just doing it over and over in frame by frame with slight variations in %/timing/etc. and looking for patterns. I'm gonna be a little exhaustive because this is a convoluted scenario. Summary: Fox can always get you but you can make it hard for him.
Fox's grab is active on frame 7. He can upthrow on frame 9 and you'll be airbourne on 13. This is less than our benchmark 15 for consistently reactable. This means that in order to full DI the throw you need to react to the setup that got you grabbed before the grab even starts.
Obviously DI behind fox is (in most cases) by far the best. It can require fox to do a smash turn before he jumps in order to catch you before you can jump out (over a % range) which is a difficult input without dedicated practice. Even top level foxes will occasionally flub and you will be able to jump out of hitstun for free.
After the throw some foxes (notably swedish ones) try to start the upair early enough that only the second hit will connect i.e. you can't SDI out. If they do this correctly then you're screwed but it is difficult.
Hitlag on the first hit of upair is ALWAYS 4 frames. Despite what some (coughcough hungrybox) may say it is not harder to SDI upair when you're at lower %s. 4 frames (technically 3 because you can't SDI during the last frame of hitlag) is a really tight window though and you have to predict the timing of the upair to some degree to get an SDI input.
There is no real benefit to ASDIing in a different direction so don't bother with your C stick.
It is possible for fox to space his double jump upair right up inside of you so that a single SDI input in any direction won't escape the 2nd hit of upair. However, this is really hard and if fox is slightly to one side of your body you can usually escape. This is normally away from where he's coming from. s0ft: "We tested it with puff at 52%. Fox up-throws facing to the right and puff di's the uthrow behind fox (to the left). We basically did several timings with fox up airing - As fast as possible (true combo) and two different delayed timings - Against the true combo up air puff can pretty easily smash di to the left because it is really hard for fox to get on the left side of her when he uairs if he is doing a true combo. Vs most delayed uairs you have enough time to jump and air dodge. So if you smash di to the left anticipating the true combo uair and then mash jump and airdodge you can get out of most uairs when di'ing behind fox."
You can also use MSDI to always escape the upair.
Because the upair timing is somewhat variable you will probably not be able to MSDI every upair but you should be able to get some-many of them, esp because foxes don't often deliberately mix up their upair timing. I suggest going from NNW to NNE, as it is the most natural movement, will work against all common upair spacings (including if you miss the DI on the throw and go straight up), and if you mistime it and only get 1 sdi input then up is what works the most often. To reiterate, in order to get MSDI you must input NNW and rotate the stick past NNE all within 3 frames. That's doable but hella fast (1/20th of a second), so practice being as fast as possible.
At low %s your hitstun is low enough that if fox is late to jump or start his upair then you can rest him before the upair comes out.
Jab Freakin Upsmash
This is a bit of a micro tactic because it's not strictly reactable and is something you either have to predict/feel in your gut.
Lots of people say to SDI the jab away but I don't like that 'cause fox can still get you at the %s where he goes for jab upsmash so I found something potentially better. If you can SDI it out then you should ASDI it down instead.
Jab has 2-3 frames of startup (2 if hits with body, 3 with fist) and then 4 frames of hitlag. if you press down during or before these 6-7 frames then you will either duck under (lmao), CC, or ASDI down the jab. It will most often be ASDI with 4 frames of landing lag. CC hitstun varies by % but is comparable.
After the jab fox has 13 frames of endlag (including IASA), so the earliest he can upsmash is on frame 20 after hitlag. This is more than enough time to grab him if you're mashing Dan Grab and now the tables have turned biiiiitch.
Ballsy AF and exactly as possible to be consistent with as DIing a jab.
You could just press down, shield the upsmash, then rest OoS but then you run the risk of him waiting to hit confirm the jab and then grabbing your shield.
This only works until 114%. At 115% fox's jab will make you miss tech if you ASDI it down and had you had time to input a tech you would have shielded. At this point DI away and spam jump+airdodge (you can't airdodge out of tumble so you have to jump out of tumble then airdodge quickly).
It will not work if you are airbourne when the jab connects unless you are close to the ground.
Note: CCing in anticipation of a jab opens you up to being grabbed.
I don't have any magic for this one just watch out for vertical spaced fox lol.