OK, point one is that IF it was a relief, if you have other arrangements than a letterbox as most people have, then he may not have seen where the box was. I know it sounds silly, as YOU know where it is, but to a postman delivering to addresses they don't know, just WHERE you want the mail isn't obvious.
Point two even if it was obvious, the legal requirement is to deliver the mail to the boundary of the address, and that was done. Strictly speaking, post people are not obliged to deliver if delivering means entering the property in any fashion. This is for health and safety issues as they are only insured or covered for accidents etc IF they comply with regulations. I've had a situation myself when I went sick after putting my back out whilst in a porch and told in no uncertain terms it was my fault because I went into it.
Regarding the missing letters, this could be a genuine mistake rather than anything sinister. Whatever the chances of trying to find out for ordinary mail are non existent.