So I don't have much storage on my device, and I'm planning to do runs that might take a long time. I know that my partner has a better device than me and can record them. Is my partner allowed to submit a co-op speedrun instead of the person who created the world?

My partner is on PC, but I am on mobile, if he records the run can he submit it in the mobile category?

You have to show the settings of the world when it's created

ok thanks

Are you allowed to play on PC to get a record on mobile though? That sounds cheaty to me, but whatever, if the rules say it's ok, I guess it's ok. Seems most logical to have a multiplayer-run in the category of the person with the "best" gear for the job so a run where 1 person is on mobile and one is on PC, it would automatically be put in the PC-category.

the run goes on the platform of the host.

*there is an exception for glitched runs, if there is a mobile player present then the run goes under mobile no matter what

yeah, on glitched runs the mobile player is forced to be the host, but otherwise anyone can host and it will go to their platform

People who play on phone, do they cast the screen to a monitor and connect a keyboard and mouse to the phone?

Different runners do their runs differently. That being said I don't think anyone would play using mirrored display on the PC as from my experience there is a very noticeable delay.

I was thinking a setup like this one:

Wired connection to HDMI-input and keyboard/mouse. Shouldn't have delay.

Well its the pocket edition of the game, and you're not using any banned input methods, so I dont see why it would be illegal