I think you can see most of those pretty quickly. For example, there isn't much to the Cathedral and Post Office. Myself, I spent a longer time at the Ben Thanh market and finding some nice restos to eat.

I have heard this from many folks - the Mekong Delta tour is a literal tourist trap and not worth the time. However, no personal experience at this one.

It depends on what you want, but make sure you spend enough time to "chill" by walking around the streets and stores. There are a ton of shops and interesting stuff to see around the core of Ben Thanh.