These promos usually last until the weekend is done if the weekend is when the sale is, otherwise typically 4 days to a week and yes, your phone will auto choose the appropriate network in 99% of cases.
Whenever these "flash sales" come out, it's imperative that you get it before the weekend ends typically. The carriers all usually follow the cadence to leave byod plans until the end of cyber Monday during black Friday, then discontinue them the fol...
Not to my knowledge. If you don't want to switch, you can keep your existing rewards. I used to be an old customer of public mobile, came back and that's what it told me upon switching to a new plan.