I'm a little hesitant to deal with this but I did want you to have a response.
There are other sites which contain detailed legalese regarding these issues. I suggest you look at these. This is no more than my opinion and I'm in no way qualified.
All of this is predicated on the assumption that you did not build with an intention to sell.
The government suggests that the name and serial number be on it.
Flats are not serial numbered.
The original trigger groups should be replaced as compliance parts. I do not understand your question regarding containing but not using original trigger groups.
If you bend a flat planning to sell it you will need to be licensed as a manufacturer.