You could use the Amiga on it's own with the TFT monitor with an RGB adapter (SYNC dongle)using VGA/DBLNTSC screen modes but it would only work for workbench and OS legal apps. It wouldn't work for most games.
That TV adapter as you say only does composite (for the Amiga side) and so the picture quality will be low.
If you have one, you may as well use the TV input on your PC gfx card as a free scandoubler.