Had a look around for images but havent seen any with what appears to be a small M12 hole. Usually a lot larger to accept MT2 or more likely MT3 with an internal M12 thread in the collets.

Certainly the slots are for tensioning end float but usually on underside.

There should be numerous T slots to use as fixings. Using MT as a means of centring isnt accurate in the slightest, its just the easy way out.
More substantial to bolt either direct to table with long screws or via an adaptor like this.

Table here is like mine with 4 T slots, makes adding a chuck on harder. Drilled through chuck and into RT and tapped, its solid and gain extra height.

Generally you centre the job to the machine. To centre table assuming it runs true (it may not) just take a reading on outside dead centre (i prefer these cheapies to any other ) and split the difference, job done. Wont work with a chuck on you take measurement from protruding round held by the jaws.