The effective optical path of the Baader Amici is 80mm. Keep that in mind. The TSFlat2 should solve your FC problem so long as you can position the FF 111mm in front of the eyepiece field stop. You have up to +/-5% slop for visual. From the TS website:
The recommended distances from the M48x0.75 thread to the corrector
In principle this rule applies: the shorter the refractor´s focal length, the longer the working distance to the sensor has to be.
♦ focal length < 450 mm: 128 mm
♦ focal length 450-490 mm: 123 mm
♦ focal length 500-550 mm: 118 mm
♦ focal length 560-590 mm: 116 mm
♦ focal length 600-690 mm: 113 mm
♦ focal length 700-800 mm: 111 mm
♦ focal length ab 800 mm: 108 mm
An underrun or an overrun of the distance of up to 5% has no visible effect on the sharpness in the field of sensors with formats up to APS-C. With larger sensors, the tolerance is reduced to 1-2%.
In many cases it's enough to simply screw the TSFlat into the filter threads in the nosepiece of the diagonal. If you can't get within +/-5% of the distance spec, then you can use threaded spacers to adjust. The 21mm Ethos has its field stop recessed 0.38" (or 9.652mm) above the shoulder, so your Ethos + Amici effective optical length is almost 90mm compared to using an eyepiece with the field stop at for below the shoulder.