FYI the tables you dropped still uses space but they become temporary segments. In order to free them you have to make tablespace read write and bounce the database so smon can free them

regarding adding columns, its just a data dictionary operation, in fact nothing has been done in the read only tablespace that´s why it does not gives you error, obviously to use that column you would have to enable tablespace in read write mode again

coalesce again does not alter the objects in the tablespace so it is allowed

DML is not allowed because you are physically writing to the datafiles