A comment, as you have solved your problem already:
I have a TS-480SAT and twin TS-590S setup.
But the settings for the keying of the IC-7100 should be similar.
In the Control Unit I have set:
Serial settings
COM3(Extra) mode (USB-COMFSK): Mode 8, Winkeyer
This way I can use the F-Keys of the N1MM log program to send CQ etc.
Windows assigns a COM Port number when you use the supplied USB cable. I use this port in the N1MM log program for WinKey and the F-Keys to call CQ etc. This works perfectly.
IO settings
IN0 Mode: Keyer
OUT0 Mode : Keyer
OUT1 Mode: I/O
OUT2 Mode I/O
USB RTS as PTT COM0
USB DTR as CW COM 0
On the Radio Unit I have set:
IO settings
OUT 0 mode: I/O
OUT1 mode: Baudot/RTTY/45
OUT2 Mode Keyer
I connect my paddle directly to the Control unit and at the radio Unit I have a twin 3.5mm jack cable to connect PAD/KEY with the KEY input of the TS-480. This input is for a straight key. You have a similar straight key input on your radio, see page 1-14 of the IC-7100 manual. In the keyer set mode, keyer type menu, choose straight Key , see page 1-13.
You can have a look at my screenshots on this site:
www.qsl.net/hb9bjl73 de Christian HB9BJL