Well glad you decided to pick up Robin! They have a bit of a learning curve, but they're very fun when you get into them.
The first step is familiarizing yourself with how many uses you have with every one of your item reliant moves.
- Neutral Special has 20 uses, Thunder costs 1 use, Elthunder costs 3 uses, Arcthunder costs 5 uses, Thoron costs 8 uses
- Nosferatu has 4 uses
- Arcfire has 6 uses, the "gentlemen" jab finisher costs 1 use
- Elwind has 9 uses, every second you hold Robin's wind jab uses 1/2 a use,
- The Levin Sword has 8 uses
You should always try to keep track of how many uses you have left on your items, but you can tell if an item has a low amount of uses left if it's flashing. You shouldn't rely too much on looking for flashing items, but it can be very useful in notifying you when Robin is about to toss them.
- The Thunder tome will flash when it has 5 uses left
- Nosferatu flashes with 1 use left
- Arcfire flashes with 1 use left
- Elwind flashes with 2 uses left
- The Levin sword flashes with 2 uses left
While all of these are important to pay attention to, I would suggest making particular effort towards keeping track of the uses left on arcfire and your Levin Sword. If either is unavailable at a crucial moment where you need to apply pressure then Robin loses a ton of momentum.
This is just all the basic data to keep in mind for item uses. Just as a personal recommendation: Because you mentioned you're new to Robin I wouldn't focus a tremendous amount on knowing
exactly when your item is going to run out of charges. If Robin loses any of these it's not the end of the world, so try to focus on just getting the feel of Robin down for now. The flashing mechanic exists for a reason. I'd be surprised if any Robin player kept meticulous track of all their Elwind uses. Hope you have success playing everyone's favorite tactician!
This is a week late, if you already found answers to your questions or you moved on from Robin already then I apologize.
To answer the first question (How does Robin deal with characters that can switch from heavy defensive to constant aggression on three seconds?)
This is one of the biggest areas Robin struggles in. He has problems being aggressive in neutral, and he also has problems countering extreme aggression while he's in a disadvantaged state. Characters that can safely play defensively, then switch to offensive play on the fly are definitely a problem. I'd be lying if I said I knew exactly how to handle this myself, but my best advice is to remember all your options. Arcthunder is an incredible move, but it's not always the best at neutralizing heavy offensive play. Pretty much every guide you read will tell you to charge Thunder as much as you can, but sometimes, if you know the opponent has an offensive playstyle on a fast character like sonic or fox, peppering them with an uncharged neutral B can be an effective way to keep them from getting in.
Other than that, jab and nair can be good anti pressure options. If Elwind hits then it can work as a mixup for escaping pressure from below.
I pretty much answered the second question up there ^^^
For the third question: All of Robin's aerials are good. I'm going to say that again for emphasis. All of Robin's aerials are good. What makes them good, however, is knowing when you should be using them. I'll briefly go over each one to the best of my ability. All of these are taking into account that they're the Levin versions
As you mentioned, Bair is very good. The disjoint is fairly large, the speed, angle of the attack, and power are all incredible for a variety of uses. In addition to standard bair uses (ledge trumping, intercepting people who try to overextend, etc.) Bair is also incredibly good in neutral. It autocancels off of a short hop; due to its safety you can throw it out quite a bit.
A lot of people consider Fair to be better than Bair for a few reasons. One of them is that both of them can autocancel off of short hop and are good tools to use in neutral. Both of them have about the same horizontal range, but Fair has a better arc for applying pressure (although bair is still more useful in specific scenarios)
Fair will combo into itself at low percents, and it deals good damage.
Upair is Robin's best aerial in my opinion. It has tremendous knockback, and an extremely wide arc that it travels. It can be used after a Fair more often than not, and of course after a down throw for a kill. It also hits surprisingly low in front of Robin. It can be used to catch some people off guard with "aerial footsies" The weak hit (at the end of the animation just below and behind Robin) can combo into a strong upair or a bair depending on DI
Dair is the most niche aerial of Robin's, but it is still incredibly strong. You should pretty much only use it off stage if you see them recovering low. It looks like it hits in front of Robin, but it doesn't. Elwind is more reliable for a spike. Dair is a little less safe and a little more offensive. Nothing much else to say. It's just a solid, strong dair.
I haven't found the absolute best scenario to use Nair in, so maybe someone else can extrapolate on it for me. Nair is good for gimping due to the angle it sends people at. It has a good disjoint, so it's good for hitting around Robin's flanks, akin to a downsmash. I can't really say much more on it, and I wish I could. Nair is definitely really good, and should be used semi often.