The Canadian and US electrical codes recognized two classes of grounding. Equipment grounding and system grounding.
Grounding the corner of the 240 volts is a system ground and is subject to different requirements.
There is a different table for conductor sizes and some more stringent requirements for installation.
The reason that the contractor proposes to connect the phase to the building ground is that it would not be allowed ground the system by connecting the phase to the grounded transformer case.
Grounding the system may not be required but it is good practice. There are some issues with drive ratings (voltage) on a corner grounded system but the same issues apply to an ungrounded system because you must consider the possibility of the effect of a ground on the system.
It sounds as if your contractor is on the right page. I would go for the corner ground.
You may still be able to justify a change to autotransformers.
for a 100 KVA 3 phase load, The phase current at 240 volts would be be about 240 amps.
A single phase transformer rated for 240:24 volts and boost service (often no extra cost) will boost 208 volts up to 229 volts, adequate for 230 volt motors.
Only the 24 volt winding will carry the 240 amps so the transformer must be rated for 5.76 KVA.
You are looking at two, 7.5 KVA transformers instead of one 100 KVA transformer.
If you need a little more voltage, look for some 240:32 volt transformers. They will get you up to 236 volts. 7.68 KVA load, so use 2 x 10 KVA, transformers, still a substantial saving, and you are over 98% of desired voltage. You will have uneven phase to neutral voltages on two phases but you are not using the neutral so no problem. This is no longer a separately derived system so your existing grounding system will be usable.
I once worked for a company who had two power systems in their plant. 600 volts and 480 volts. They had equipment rated at 575 volts and 460 volts. It seemed that every piece of equipment that came in was the wrong voltage for the feed at the location. We had pairs of small dry type transformers all over the plant, adjusting voltages for individual machines. We never had a problem.
respectfully