I should have said I am concerned about the form factor, since the 18650C holds the M5Stick INSIDE its case.

I checked the size of each M5Stick product and they are the same size i.e.
Product Size 48.225.513.7mm.

So the answer should be yes.

Thanks anyhow, ajb2k3.

Is the M5Stick 18650C product suitable to power the M5StickC PLUS2?

With a M5StickC+2, is WiFi communication possible and reliable?

It should be possible since the included ESP32-PICO-V3-02 chip has WiFi built-in.
However I read a few (old) posts where problems were reported with earlier models.

In addition, there is no external antenna or antenna connector, surely impacting the range covered.

Have you add success? Which programming language was used?

Potential buyer.