Can I use cornstarch instead of flour in a bechamel sauce

Yes you can. It ia alot smoother than flour with a better flavor all around, in my opinion.
Cornstarch and cornmeal are 2 different things, so it won't be grainy or taste of corn at all.

The texture can also vary. Cornstarch tends to produce a somewhat "gummier" sauce.

Yes, as well as any other flour substitute, but the taste will be different. Cornmeal- corny flavor. Arrowroot, ect,