What is special about 1x11 - why not just big cassette and single front ring?
SRAM have emphasized the 1x configuration, but 1x systems can be built from Shimano components too.

You are right that a 1x system is pretty much a wide ratio cassette and a single front ring. Double-wide chainrings are commonly used to help chain retention as there is no front derailleur cage to guide the chain. Clutch derailleurs are also used to tame chain slap on rougher trails encountered when gravel riding.

You can use a double crankset with a single ring on it, but it's a hack and as you say you will have cross-chaining problems. Dedicated 1x cranks align the ring with the middle of the cassette so the chain deflection is minimized.

You will not be able to use a XT MTB derailleur with a road drop bar shifter. Shimano MTB and road 11 speed use different derailleur actuation ratios (see here) so MTB derailleurs and road shifters are incompatible. The largest cassette that Shimano long cage road derailleurs can take is 11-34, but, if you are coming from single speed that is going to feel like a huge ratio range.