There is no need for any registration, there is no limit and everyone is allowed. OkPay is supported payment option. Time delays vary on multiple factors in trades. (bitcoin backlog, other user you are buying from or selling to,…)
bitsquare is a java application for windows, linux&mac. i dont think it runs on android.
you dont need an account. its runs peer to peer.
yes, okpay is an option for sending/recieving local currency.
nigerian naira does not seem to be supported inside bitsquare - yet - ask our prince to add it. (maybe you can use another currency via okpay?)