why is 'I shall' wrong ?
It shouldn't be, I'll report it just in case.
... Let's get down to business to defeat the Huns.
LOL
"I shall be" was rejected as incorrect. Is it not the case that SHALL is the correct from to use in the first person both singular and plural? Most native speakers use WILL but SHALL may actually be the correct form.
That's certainly the correct (but as you say, often not used) form in British English
mazel tov