The only definitive legal opinion which can be given on a matter of EU law is that which is given by the European Court of Justice. There is already a great body of expert legal opinion which has been pointed out to the Scottish Government/SNP confirming that its position is not legally sound. They've merrily ignored it. They would happily ignore this too.Effectively what you're saying is that the Uk govt should deny voters in Scotland the chance of hearing a definitive legal opinion on a crucial issue because doing so will increase their chances of winning the referendum. Personally I would rather give voters all the information available so they can make a fully informed decision one way or the other. As for us asking the question, the EU have made it clear it has to be asked by the Uk Govt.
Secondly, if this is legal advice to the Government, it would not be publicly released as standard. That could potentially happen under a small number of circumstances.