Of the options you have there I’d definitely go for the inline coil for £200.
I don’t have an enduro 650 but I’ve ridden them with the inline air, monarch and ohlins shocks. The ohlins is (unsurprisingly) far and away the better shock.
I’ve only ridden a piggy back double barrel and not on an enduro but if the inline is anywhere near to that then it will get you close to the ohlins performance without the cost or weight, and you can set it up exactly as you like.
I’ve had a couple of enduro 29ers with monarchs and they are pretty good but definitely not worth £400. You should try returning the inline as specialized have replaced a couple of mine with monarchs on warranty. I’d still choose an inline coil for £200 though.