Can anyone suggest any other reliable merchant like coinhouse where you can purchase BTC with Nigeria Debit cards.
I'm not so sure but It may be because of the recent restrictions placed on Nigerian Debit cards and as a result online transactions outside the shores of Nigeria does not work. Tried purchasing a domain recently and it's impossible. If You're still looking to buy Bitcoin with your debit card, I suggest you do so through Localbitcoins & Pay the seller with your debit card via a payment processor like quickteller.
That's just how I make my transactions over here in Nigeria