Strictly Google Sitemap is a feature rich Google XML Sitemap plugin designed with performance in mind for large sites.
If you rarely add articles to your site then the "Rebuild on Post Save option will suffice. If you reguarly import articles from feeds then you should use the "Rebuild at scheduled intervals" option and then either setup a CRON or Web CRON job or use the inbuilt scheduler system to set a rebuild period. If you import articles once every hour then rebuild once an hour.
This depends on the size of your system and the performance of your server and is best found out through trial and error. If you are not using a Sitemap Index and all your articles are within one sitemap then the "Build and Join XML with SQL" option might be the fastest as it will not need to split the records up into multiple files. If you are using multiple files or have a large number of records then the "Build XML with SQL (join with PHP)" option might be better as the records can be split into files during the build process.
Yes this is true it doesn't.
This is a known issue and seems to be related to that on some systems the "blogurl" option is populated and others it is not. Therefore my system uses the "siteurl" option to determine the output location. Changing references in the main class from get_option('siteurl') to get_option('blogurl') might fix this but I have not been able to test this.
Sorry but tough luck. I wrote this plugin for my own requirements not anyone else and if you have conflicts with other plugins or require extra work then offer to pay me to do the development or do the work yourself. This is what Open Source programming should be about. I wrote this plugin as other Sitemap plugins didn't do what I wanted them to and you should follow the same rules. If you don't like this plugin or require a new feature you must remember that you have already bought a good amount of my time for the princely sum of £0.