I think that any self-respecting poly person would have replacing the primary about the furthest from their thought processes. So as long as the person is really up-to-speed with poly then you shouldn't have to deal with at contingency. If she DOES, then you need to end it, because that WILL all end in tears.
Respect for the other relationships is absolutely key making poly work - and that goes all ways.
The trick is not to make the person feel like second-best (because let's face it, what self-respecting person would want that?) - it's not a ranking game or a competition, it should be about love. Allowing the love to grow naturally, without proscribed paths, actions and limits, is going to make people feel at their most comfortable.
That's not to say that everyone involved can't have boundaries of acceptable behaviour, and there have been many threads on here about negotiating those, but at least the ability to let the heart roam free is a very good feeling for everyone.
It really does sound like you are approaching this by communicating well - there are always going to be rough moments and fears, but it sounds like you are building (or have built) a framework for working them through.
"Listen, or your tongue will make you deaf." - Native American Proverb