For normal applications, they usually don’t even contact the guarantor or references. On the other hand for urgent applications (24h) it seems that they often call them.Same here, I'm in a bit of a planning phase with oath coming up, had the same questions about passport app. The person I will use as guarantor is going on vacation soon, so I'll have to get him to fill out his part before hand, then the certificate # by hand after oath. My two refs will hopefully be in town also. December being holiday season, people are generally out of the country or traveling somewhere, what happens if they cannot reach a guarantor or reference? Does that delay things, or cause any complications?
For the references, you actually can specify more than 2 if you feel that some may not be available. I got this information from the passport office. I guess that you can print an additional page of the application form just to fill 2 additional references.
But you can have only one guarantor, so you need to make sure he his available otherwise you might want to pick someone else.
Finally, I think that you should warn your guarantor and references that they may get a call from the passport office.