Hi Jack,
There are lots of places to send an email through Method, but if you are contacting customers, the activity screen is the place you probably want to be. So you're in the right spot.
When you say you've changed the default in the From Name field, do you mean in the Send Email action? If so, I think what you're seeing is the screen actions placing the User's Entity name into the From Name field. This action is found on the "Send an Email?" checkbox on the screen (see below):
What you can do is change this action from grabbing the Entity name to pull the Company Name from the Company table instead. Now its usually bad form to hard code values (like the recordID = 1 below), but you will only ever sync one company to this account. A better way to do this would be to add a second action that will grab the recordID from the Company table and put it in an action result. Then use that action result to locate the record. For an example of this, see action #28 on the Save button of the same screen.
Please let me know if you have any questions. Hope this helps.
- Adam