Generation of alerts if real work is not entered is not sending from the email as set in Global Parameters, is it sent from the user who created the activity?

I don't catch what you deal with : alerts on real work not entered sends email from CRONNED process, and for several activities, so there is no link with any user creating any activity.

Would I simply be able to go in to the DB table "activity" and set the activities I want with the "idUser" using SQL as the id of the resource that I would like it to send from?

No, you should never change data directly in the database : it may lead to unexpeted side effects.
But you can change creation user in the title area (where creation date and last modification date are displayed) if you get the correct specific access right.