with the swicthing jack i show on the schem/layout, you can use the amp as a combo by removing the plug and reconnecting the speakers. no dummy plug needed.
as for the mid control, what i did was remove input 2 and moved the volume to to that hole, and rewired a mid control in the tone controls by sliding the bass knob where the volume was and placing the mid control where the bass was. this way when used as a combo the only function i lost was the use of input 2, which i never use.
Disclaimer: I make and sell Buffers. I also sometimes Modify, Build and sell preamps and amplifiers. My opinions are not intended to be sales pitches. I am a one man shop and prefer to spend time with my wife and family, i work full time on call and love to spend my spare time doing other things. I only make, modify or build things for those that seek them.