Does the welcome coupon go to customers I add myself or who check out as guests?

No. The welcome coupon is only sent when a shopper creates their own account on your storefront, by signing up or with a social login.

These do not trigger it:

• customers you add under Customers → Manage Customers,
• accounts created automatically for guest checkouts,
• newsletter sign-ups.

Also check Marketing → Welcome Coupon: Send a welcome coupon when someone signs up must be ticked with a value greater than 0, and you need to click Save. If your monthly email allowance has run out, the email cannot go out.