We renewed the policy over the summer and went over it very carefully then and yes we are covered if in the house, obviously not if we went out leaving the house unlocked.
Depending on time of day, temperature etc the second door is shut so once dark or once everyone is home from school etc and that is locked though you could break in by smashing the glass and then putting your hand through to open the lock iyswim. As a said during the summer the doors are often open if kids going in or out or as there will be this summer baby asleep in pram (unless they are like ds1 who never bloody slept...). Bizarrely the back door is more likely to be locked as we go into the back garden less especially in the winter.
Anybody in our house can see a person coming to the front door, I wouldn't see them.if I was upstairs in bathroom.but if I was taking a shower or bath then I would shut the second door which locks, just leaving front porch/hallway unlocked iyswim. There are seven of us though so lots of people milling around in the house which isn't that big so windows, doors to the front are visible it's not as if we could be in a back room and not see someone approaching the front door if that makes sense. Ie now I am sat in living room and can see out front, as is ds1. If someone came in now they would have had to come up driveway without being noticed then open front door, then the next door into the house where upon they would fall over dd (5) and Ds4(7) who are currently sliding around the hall entrance/dining room in cardboard boxes, plus dh is in the kitchen sorting dinner and kitchen windows looks out the front.
As I said in other houses with a different layout in other areas I kept the door locked one where front door opened right onto Street,another where the design of house and Garden meant it felt less safe. Big hedges, walls, quiter area just much easier for someone to come to the door without being noticed.
It's an interesting discussion and there are lots of factors that come into play. We all make different choices and risks assessments.
Regardless though the delivery driver should have knocked,mine would knock and leave in front porch, but they know I am Ok with that.