Sentence Structure Problem

Hello teachers! This is Raymond.
Recently I've been reading the book 'Murder of the Orient Express' by Agatha Christie.
However, there is a sentence in the story synopsis which I don't really understand its structure.
Here is the sentence:

A passenger lies dead in his compartment, stabbed a dozen times, his door locked from the inside.

• Is the part 'stabbed a dozen times' in between the two commas a kind of reduced relative clause?

(the active voice one - the passenger [who is] stabbed...)
If yes, am I correct to write the verb 'is' here? It's because I've been told that story synopsis should be written in present
tense.

• Is the clause 'his door locked from the inside' grammatical?

It seems weird because it appears as an independent clause to me. I just don't understand why a comma (I mean the second comma, the one after the word 'times') is used here to connect this clause to the previous clause. Shouldn't we use the word 'and' or simply use a period to separate the two parts? (by two parts I mean 'stabbed a dozen times' and 'his door locked from the inside')

I'm really confused about this sentence.
Thanks in advance!