Wouldn't nigeria need to spend some of the money it receives from exports domestically? Most countries do and I don't see any reason why they would not. Also I don't see why they would need to keep the money they receive from exports in bitcoin even if they did not need to spend it as bitcoin is very unstable and is far from certain to increase in value
I just computed how much should be worth a BTC if Nigeria want to be paid in BTC and obtain the same return from exported goods.
Obviously, if they accumulated BTC they do because they want spend them. it is obvious the security of the network rest on the value transferred by it. If Nigeria hoard all the BTC and no value transfer happen, the miners will start to do something else, as the reward go down.
If they pay people with BTC obtained from imports, these BTC will flow back in the general economy in large extent and flow out of the country, in a dynamic balance.
This is because I computed a case where the IN and OUT flows are equalled and Bitcoin is used just as a payment network and no balance is kept.
I didn't argued about the pros or cons of the Nigerian government keeping some balance of BTC. It would have some advantages and some disadvantage for them.
IMHO, if the Nigerian government require BTC for payment of its exports and imports, the exchange rate would shoot up ten times or more.
At that level, the stability of the exchange rate would be a lot higher, because would be a lot more costly to move the price up or down.
Remember they would need someone to buy 13K BTCs every day (and deliver immediately) at 1200$/BTC, they they pay back the imports.
13K BTC every day is 4 time the mining at the current rate, 8 times after the next halving.