Not only did the Edinburgh Agreement indicate that the UK would honor the results, but the UK government also explicitly granted the Scottish government the right to hold the referendum (constitutional issues would normally be a power reserved by the UK parliament).
Parliament may not bind its successors.
The government isn't going to change before the referendum.
The principle doesn't just apply to changes of government, it means that a commitment by parliament is only good so long as parliament consents to that commitment. It can be changed or overruled at any time.
Sure but for a Government being able to function depends on you being able to take their word to have at least some value. They have committed to this. I think it is reasonable to assume they will stick to it. Yes governments break promises, but I think this is one they will stick with.