Any decent sized installation will use current clamps at the local transformers to scale the charging rate up and down depending on other users in the local area.

Ie. in the evening whilst everyone has their ovens on, charging might only be 3 kW per bus, but then at 1am when everyone has gone to bed, it can be 30 kW per bus.

Using that approach, you can get far more capacity out of old infrastructure.

Unfortunately, some utility companies aren't amenable to that approach, and instead insist you pay to upgrade the infra, since to them it's a free upgrade.

Not really. Buses need to be charged whenever, not just overnight (it creates enormous logistical problems otherwise). A typical bus route running 5am to midnight say is not going to last with one charge, depending on length.

Also, if it is a cold night and everyone leaves electric heating/heat pumps on, what happens then? Noone can get to work the next day?

Regardless most urban transformers are not going to have 15MW of overnight capacity spare even on a good day. The largest LV substations might be 30MVA in the UK at least - they won't just have half capacity suddenly free.