Will IBKR's commissions and fees change when my account is migrated due to Brexit?

No. IBKR commissions and fees do not vary by the broker your account is maintained with.

Is my account restricted from being transferred to another broker if I have an open Intraday Margin Deficit (IMD) despite being in end of day margin compliance?

Dividends and distributions: If I own foreign securities in my IRA and there is a foreign tax withheld, may I claim a foreign tax credit?