I am sad to say that this is a not fun job even with a bare block on an engine stand.
there is no opening of that heater tube into any portion of the water pump housing.
yes the tube is pressed in and they are famous for doing what yours has done. With a bare block I understand it can be tapped to accept a steel/brass pipe nipple but that is something certainly best done on a bare block project where metal shavings can be properly dealt with.
The ones I have removed have been on bare blocks so I have no knowledge of what the best approach might be to capture the inevitable pieces of metal the process will result in.
sorry I can’t deliver some wonderful news.😕
hopefully someone else will post their magic solution to make your task easier.