Yes. One Yodlee subscription covers the QuickFile account rather than individual banks. So you could even have one subscription for HSBC, Barclays, NatWest, and more.
Yodlee does have strong security. We have a knowledge base article dedicated to this here
Unfortunately, this isn't possible. You may however wish to look at our Google Chrome plugin. This stores all your details in chrome itself, so they never actually leave your computer.
This plugin doesn't support as many banks as the Yodlee feed, and it isn't automated (it requires you to access the page and click a button or two).
The other thing you may wish to consider, is some banks (I'm aware that HSBC does this) allow you create a new user account to access the accounts, but you can limit what they can do (e.g. disable transfers, limit what they can see etc.).
The log in details would be specific for your bank account, so your QuickFile account details are completely separate from this.
What type of RBS bank account do you have? I'm not familiar with RBS, but looking at their website, they offer the choice of Bankline and Digital banking. I've just done a quick search on Yodlee during the set up process, and the one that's listed as most popular is Bankline. If you do a search however for Royal Bank of Scotland, it will reveal a few options, including the two I mentioned:
I believe you may have a Digital account (although I could be wrong), as this seems to ask for a customer number, pin and password when trying to set it up.
Let us know how you get on, and if you need further help, please don't hesitate to ask!