I don't know for certain, but I have a suspicion that the "bruising to interior chest wall" may have been a journalistic error or mis-hearing. If the inside of the chest wall is bruised, by external trauma such as a kick or a punch, then I'd expect there to be at least the same amount of bruising externally too.
I suspect that it may have been anterior chest wall - that is, the front of the chest.
Unless of course the external bruising was so badly decomposed after 11 days that the only evidence of the bruising was on the inside... another possibility.
But a kick to the chest is something that I wouldn't really expect - it's not that easy unless you're into martial arts etc - and even then getting the balance right isn't easy. A punch would be much more likely. Or a knee.
And also I find it interesting that no mention was made of any rib fractures - yet for there to be bruising on the interior chest wall, I would almost expect rib fractures to be present as well. It's possible, but highly unlikely, to cause bruising on the inside without at least a cracked rib or two which actually causes the bruising - i.e. the bleeding and swelling. Also known as a contusion.
I really would like to see that autopsy report when it becomes available...