Evening,
I've reviewed your website, although can't read much in to the specific error's as they are hidden. I've also read through the outline document, thank you for taking the time to write it as the information may come in useful if some of steps 2-3 need recoded.
In short, I'm a fairly competent PHP developer with several years of experience in both PHP and the Zend Framework, also familiar with the Google APIs, including the Calender one, and the various forms of integrating data between calendar/scheduling applications. I'm confident that I could quickly have your scheduler back up and running, either on the full zend framework if required, or most likely just the single required modules needed to run the software. Failing that I could replace the Zend specific functionality (probably 20% of the script) with custom PHP 5 code to deliver the same end result.
At this early stage I can say that I'm free to start working tomorrow circa 1300 your local time, should have everything identified with a plan to move forward circa 1-2 hours (max) after getting access to the website/code, and from there it'll either be same working day if it's a Zend upgrade with a few nuances, or the following day if recoding is required, in order to give time for both my testing, and your own acceptance testing.
Many Regards from Scotland,
Nathan
ps: If you reply tonight, I may not reply until tomorrow am, as it's very late her