Hi Richard,
If you calmly follow each step outlined in the docs and don't skip any steps or go most of the way and change something (attach the app to a different Apps account), there should be no issues. If you'd like specific 1-on-1 support for setting up the Custom Domain mapping, you should post to stackoverflow and get support in that forum which is meant for that, while this forum is more meant for general discussion, which brings me to your second point: will SSL be supported in the Developers Console?
Yes, this is confirmed and is even mentioned in
the docs you would have read in order to set up a Custom Domain with SSL. The main Custom Domains SSL doc, which you should read from the top, paying attention to all details, can be found
here. The feature, as the first link mentions, should be rolled out by the end of the month.
Let me know if you have any further questions before proceeding to open a stackoverflow question regarding your specific stage of the documented steps. When you do so, feel free to post it here and I'll be happy to troubleshoot in that forum :)
Best wishes,
Nick