I don't believe potential reclearances is a part of fuel planning calculations. So no you certainly don't have extra fuel specifically in order to fly other routes (i.e. no regulations require it, unless your company does)
You really have to look at the situation from the angle of what you do once your in the air vs. planning on ground. As for calculation of ETPs, the reality is that I don't think there's a tool that could be as accurate as flight planning software. You can use the ETP circles, that'll help, and use the average winds in order to get a decent estimate. But in any case, once your in the air go anywhere that meets ETOPS requirements and that'll fulfill your needs.
You do bring up a good point and one that I haven't thought about, namely, what if a reclearances takes you further away from your ETOPS alternates. As for your contingency it might or might not cover the extra fuel required. It's not there for any specific purpose. In any case you can use it up for whatever purpose is necessary during flight (and even before). The reality is that it's up to the crew to work out (using whatever tools are available) to decide whether or not they can accept a reclearances.