|If an address belongs to a group and the group has an own template for new messages, a new message to a single address of this group seems to be automatically based on the new message template of the group.|
Yep, that's the way it works. Address book group templates are supposed to trigger when you're sending a mail to an entry in that group. Of course they're also used when sending mail to the whole group, but that's not the way they're being triggered.
When you'd like addresses to a single address to be processed otherwise than multiple addresses, you could use an if macro that calls different quick templates.
Another way, less complicated, but also more manual, is to create a quick template starting with the %Clear macro (followed by whatever you'd like to have in the template). Whenever you're sending a message and you don't like the automatic template, you type the name of the quick template and end it with Ctrl-space, that triggers your quick template.
|Is there a way to have single address mails be based automatically on account’s new message template instead of assigning a separate new message template to every single address that also belongs to a group?|
Actually that's a very bad idea. Entering templates in the individual address book entries means that the group template will never be used, not when you're sending mail to individual entries nor when sending mail to the whole group. And somehow I get the feeling that isn't what you're after.
Apart from that, it's a lot of work, whereas you'd achieve the same by deleting the group templates.