Unfortunately, I can't say much about this because we rarely heat milk with the steam heater. But it should take the same amount of time to heat up from a standstill, since it's connected to the same system. Once the machine is heated up, we can make one coffee after the other. So I think that even if you switch over or use the steam heater after making coffee, there is no additional heating time.

The warm-up time is about right. However, it is recommended (as it also says in the manual) to first run a cup through empty, i.e. only hot water. Then the following coffee will be hot enough. I don't know how to heat milk and make coffee at the same time. But you usually do it one after the other anyway...

For a machine to be able to make coffee and steam at the same time or one after the other, it needs two separate heating circuits, as the coffee water should be 92 to 93 degrees, but the boiler for steam has to heat up to 110 degrees. That's why it takes about a minute to switch from coffee to steam and vice versa in single-circuit machines, until the machine has heated up or cooled down to the respective temperature. If there is no mention of two heating circuits on the Solis at this price, it won't have that either (compare the Bezzera machines from Magica upwards... ;-)