No they don't. An optional field can be deprecated and replaced with a different field. This can be done to change the type (also some types can be changed, although you probably shouldn't).
You can deprecate an optional field and reserve the field number, but if you reintroduce use of that field number with a field of a different type that is not backward compatible. Types can be changed if they are binary compatible, but in that case they haven’t actually changed, because the binary format is the canonical format.
Right, I'm not sure what your point is. You can always add more fields, so being unable to change the type of a field isn't a problem, since you can introduce a new field and start using it. You cannot however, stop using a required field. The best you can do is set it to a nonsense value and leave a comment saying "well we need to set this to something, but we don't actually use it anywhere.
Because to be sure of that you can remove it, you need to be sure that every storage system and every piece of middleware and every since thing that links your proto anywhere in the world that you might care about is upgraded, otherwise if they encounter a new message they'll crash.
If you only have a single client and server, and you control both, this is doable. If you don't have that though, you cannot.
Required usually cannot be deprecated.