International baggage fees

A friend of mine has questions on baggage fees when his trip is split between the domestic and international into two bookings. I think people in this forum have more insights to answer my friend's question. This group saves over five hundred bucks if to split into two bookings, i.e., one domestic and one international.

My friend of five people with these trips:

Booking#1 from TLH to DFW on AA
DFW to TLH on AA

Booking#2 from DFW to ICN on AA then ICN to SGN on KE [Korean Airline]
SGN to ICN on KE then ICN to DFW on AA
Questions:
- if AA is the first booking as a domestic carrier, any free baggage even the trip is all the way to SGN with connection at DFW and ICN?
- for return route, the first carrier is KE, what is the baggage fees for the homeward trip?
- will AA charges baggage fee for the return segment DFW-TLH since it is a domestic booking?

Thanks for helping my friend's questions.