Yes you are allowed to use remixed bundles. The only current route that uses remixed bundles is Pantry.
Category extensions had a vote on possible categories 4 months ago. One of the suggestions was all max skills, but it didn't make the cut. I would recommend joining the discord or following announcements on the category extensions board in the event that there are future votes for adding new categories.
As far as I’m aware switch has the slowest load times and completely different RNG than all the other versions, making it (extra) difficult to get a competitive time. That being said you should run the game on whatever you’re most comfortable with and swap to PC if you ever find the opportunity and the motivation to make the switch (pun intended)
Audio makes a run easier to verify. There have been discussions of requiring audio, but it is not currently required. I don’t think you can go back and add audio after the fact, unless you somehow record the video and audio separately at the same time.
Animation cancels are performed by pressing right shift, delete, and R, on the same frame, after using a tool or weapon. Spam clicking is a normal vanilla mechanic. You can’t accidentally animation cancel, so you should be okay. I’m pretty sure there’s a guide/resource talking about animation cancel and linking an AutoHotkey script that most runners use. Animation cancelling does save time, but not nearly as much as good rng and game knowledge. I find that it’s easy for some players to assume glitchless runs are faster because of animation cancelling, but that’s not exactly the case (see mines 50 Glitchless vs Vanilla). I’ve done about 200 attempts of glitchless mines 50 with animation cancel and gotten a 13 minute run. Did 3 attempts of vanilla mines 50 to see how much of a difference it makes and got an 18 minute run (would’ve been 15-16 with animation cancelling). Long story short, animation cancelling makes a fast run faster, but doesn’t necessarily make a slow run fast. Thanks for coming to my Ted talk :)
I dont think anyone's tested how much faster it is, but it is considerably faster. Menuing quickly with the controller is basically impossible, mouse allows you to reach tiles around your character without moving, move the bed/tv reliably, check for forage behind trees without walking behind them, and probably a lot of other things im not thinking of. Its been a very long time since I played with a controller. I remember there being a bit of a learning curve getting used to keyboard/mouse controls, but once I got used to it I felt that playing the game overall was easier.
The only difference is mobile runs on version 1.4.5. You can fairly easily sort the leaderboards by version and watch runs that use 1.4. The only notable difference will be that PC runners use/used 1.4.0 because 1.4.1 significantly nerfed explosive ammo to require level 8 combat and cost 300 gold each at the adventurers guild. Other than that any run you find on the leaderboard that is run on version 1.4 should be essentially identical. Also most 1.5 categories have no difference between their 1.4 counterparts. If you still want advice or better understanding about why certain runners do certain things I would highly recommend asking in the stardew speedrun discord, there are many people who are willing to help there.