Please note that the word is phrase (with an 'r' in it) and not phase.
http://dictionary.reference.com/browse/phrase(1 and 2) If a phrase or payload appears in two different commands, and there are no other command parts to differentiate it from another command then there is no way to know which command will be executed, unless one command is in a group that has been disabled, or if one command is in a group that has a higher priority. It is OK if part of the command structure is not unique though, as long as the command overall (the combination of phrases and payloads) create a complete command structure that is unique.
(3) Please do not only look in the forum and in youtube when you have a specific question. There is a wealth of information in the wiki. Go to the wiki and look for groups and you will find answers to pretty much everything you are asking here. Yes groups are good for organization, and they can be turned on and off to provide context. They can have different priorities, and they can turn on and off automatically depending on which program has focus. When rebuilding and processing all the commands (during load, or after an edit), groups are processed in parallel. One thread is created for each group, to speed up load time.
(4) only enabled groups
(5) it listens for all command in enabled groups, but only executes them if they are set to "always on" or if you use a prefix.
(6) More commands, and larger payloads will use more memory, and will require more time to load, but will probably not require much more processing power once they are loaded.
One more note on Groups that is probably not mentioned in the wiki. Sometimes when a group contains too many commands with similar sentence structure, or using the same payload xml files, especially if you have lots of optional command parts, it will get confused and have trouble loading all the commands. In this case, moving some of the commands into a new group can often help. Don't ask why this happens, it is something going on in the black box of the MS speech api. We can't know what happens or fix it, only work around it.
