Trying to remove Grohe single hole faucet. There’s no obvious nut to remove, the C washer might be threaded on but it can’t turn with the pipes blocking the twist and it won’t twist independent of threaded pipe anyway. What am I not seeing? [![enter image description here]
I believe that this style of faucet is tightened from the top. The faucet will be held on to the monoblock with a grub screw somewhere on the tap (probably requires a small hex key to loosen).
The faucet should then lift away revealing the fixing that you loosen to remove the c-clamp below.
Edit: Following your edit to add the picture from above, looks like my initial suspicion was wrong. Does it have a pull out nozzle?
I think scotty is right; this faucet gets tightened from the top.
According to these installation instructions (similar, but not the exact same model), the faucet gets tightened from above with a fitting key (see step 3 in the instructions, fig. 1 in images), then the faucet body snaps on to the lower assembly. In your updated picture it looks like you have the sprayer tube removed. So now, the upper part of the faucet should be removable, but you'll probably have to work gently to unsnap it. Then, you should be able to untighten the tube from above (see E in fig 1.)