Why is the LHG mount too small? I can’t use the ratchet wrench to tighten or release. Is it too big problem to make the LHG mount 10mm longer?
Mikrotik says: please change the key 
DynaDish mount also suffers from this shortcoming. Sometimes possible to use a ratcheting end wrench, but even so sometimes there is not enough room to swing it one ratchet step. We have had to do these much more than we would like using adjustable Crescent wrenches. 
I have similar wrenches too - by Craftsman I think. But it is not so good like “circle” ratchet wrench.
Is there any reason you can’t just use a shifting spanner, fixed wrench or socket wrench?
You sort of have to… but that’s often not a very tower-friendly solution. Keep in mind that half the time, the nut is on the far side of the unit from you, where you can’t see it. On some of these mounts, you can’t even use a socket because there isn’t enough room for the socket wall, and you can’t use a nut driver because the bolt extends too far out. There’s no good reason for not designing these joints to industry standard tolerances.
The person who designed it should ever use it. On the tower you need do it quickly and easily. This way, Mikrotik never will be top. It is not only my opinion. Every technician hate obstackles like this. It is not technician friendly.