Screen buffer overrunning on Model 100
The Tandy Model 100 only has a 40x8 screen, so I need to pause frequently to be able to read much of the content various BBSes spit out. However, when connected to my M100, the WiModem will spit the entire received text, and will not allow anything to be sent to the remote server while it is printing, meaning I have to wait sometimes minutes before I can interact. This behavior continues after disconnecting from the BBS; the WiModem will deliver all the text it has received regardless of whether it is even still connected, with seemingly no way to stop it other than rebooting the WiModem. 

Needless to say, this is a big usability issue. What am I doing wrong?
That is perfectly normal.  I guess I could stop the output when the connection drops.  Otherwise, it should provide all of the data that is in the buffer to the DTE (computer).  However, serial communications are still bi-directional when data is incoming, meaning that whatever is sending the data can be told to stop sending by your computer if your computer sends such a command that can be understood by the sender.  What baud rate are you using?  What terminal software are you using?  Are you using the proper flow control method?  You need to enable hardware flow control and must be using the proper cable that can provide this.  Here is some information on the Tandy M100 serial port:

I know that the built-in TELCOM needs the STAT set to: 38N1E

You will also need to enable hardware handshaking on the WiModem232 with AT&K1.
After some more poking around, I believe the issue is that the screen writing on the M100 is so slow that what I’m seeing is the screen buffer emptying while the WiModem thinks that data has been written. I am guessing that it’s a limitation of the TELCOM program that it cannot abort a screen buffer write.

TELCOM also does not support hardware flow control, only XON/XOFF.
Configuration in TELCOM is 38N1E, any faster than about 600 baud results in buffer overrun and dropped characters.
600 baud is the max speed capable by the UART in the M100 with the built-in terminal software. I do see that 38N1E (where the 'E' is the flow control) enables XON/OFF flow control. So, you will want to use 38N1D instead.
I’ll try it without flow control and see if it’s different.

That could explain why the modem won’t respond to the +++ offline command while the screen is printing.
The modem responds, but not until after the remainder of the buffer text is output. The problem is that because we are dealing with slow serial vs. fast internet emptying the buffer on a disconnect will result in missing data that should be displayed.

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