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I have a wild idea to replace a crankset on my Merida Crossway 500 (2014) (which is now SR Suntour NCX-FX-10s 48A-36-26 CG) with Shimano 105 5750 cCompact 10sp (switch from 3-speed to 2-speed).
The question is: will my older derailleur be compatible with new crankset?
If you want to switch from a MTB triple to a road double crank you'll need to replace the front derailleur and shifter to get the correct derailleur movement.
Putting a road crank on an hybrid frame designed for MTB components is problematic:
You'd have to find a road groupset that also has compatible flat-bar shifters. 105 5700 does not, the latest lower end road Shimano groups do.
I think (from looking at old reviews of the bike) you have a Shimano Hollowtech II bottom bracket. You very likely have the wider MTB version that fits on 73 and 68mm wide shells. A road crank spindle is made for the narrower road BB version for 68mm shells only and is therefore 5mm shorter.
There is also the problem of clearance between the larger chainrings and the right hand side chainstay.
No, that would not be compatible.
The MTB 3x derailleur makes a different movement than is required for a road 2x crankset. Also your shifters are for 3x crankset. You could try to come up with a hacky-solution (modify the chain cage, try to set your limit screws to some extremes, maybe you have a shifter that is configurable between 2x and 3x, etc, etc). But in principle it's not compatible.
