Hi Marius Stoica.
1. You can create as much as you need personalization keys to transfer data between device and SUP and bind to load parameters of the MBO (as Midhun said).
2.a: HWC make you have not coding then ofcause I think there is no "standard" way to customize Error screens. you could customize custom.js file, look at this link about custom UI for container apps:
: http://www.sdn.sap.com/irj/sdn/index?rid=/webcontent/uuid/f0118416-5062-2e10-9e93-8cca742da350
or you could define mbo contain error and make it one screens in your flow.
2.b: I dont know why but when you define EXPORTING and TABLE in the same time, attributes of export table alway contain attributes (and export value too) of export structure too. you can just delete these attribute or you should define all export as table or structure, i think so.
2.c: no ideal 
Thanks and regard.