Members of the EU are expected to join the euro when their citizens agree to it. Nobody in the EU is forced to join the euro, indeed several EU members have held referendums where their population have rejected the euro.
Re currency, I have had discussions with someone far more knowledgeable than myself and I think this is what was said.
Scotland currently prints its own currency. Scottish banks hold reserves to back this currency.
The pound is a tradeable currency. This means that any country in the world can adopt the pound at any time. It requires no permission from the bank of England.
In the same way us dollars are often used as currency in areas other than the us. The caveat with this is that the value of the currency is dictated by the home nation, BOE for the pound, federal reserve for the dollar etc.
So on independence Scotland can keep the pound. There is bugger all the bank of England can do about it. In the same way Scotland could choose to adopt the dollar, the euro, the yen, or any other tradeable currency.
As already mentioned, if Scotland chooses to use a tradeable currency then interest rates would be outside Scotland's control. This is the same if scotland Continues with the pound or moves to the euro.
Therefore, it would make sense to keep using the pound as that is what Scotland currently prints and has reserves for.