I have one digression on your post. Always mind the terminology or you will get lost in it. Use the term "modifier" for words and phrases which particularize,narrow,classify,describe,evaluate,or in one word, which "modify" the meaning of adjective,adverb or noun (and only those words;you cannot modify pronouns,conjunctions,prepositions,determiners or verbs).If you put modifier before that word you can also say that it is a "pre-modifier", and if it is after the word then it is a "postmodifier".Very logical,isn't it? And that is how you make a phrase in stead of a single word.Supposedly that you are a woman or girl,which I don't know (and btw it is for the serbian language unconceivable situation since we have gender-inflected verbs so there is never a confusion regarding the gender,and also the vocabulary can hardly be characterised as sexist or something like that),I could say:
1.A girl has posted this questions. The subject is a noun (i will ignore the article)
2.A nice girl has posted this questions. The subject is a noun phrase :
3.A nice girl from Las Vegas has posted this questions. The subject is a noun phrase :
pre-modifier+ noun + post-modifier(prepositional phrase "from Las Vegas" , to add to this a citation from Quirk's grammar ".. a prepositional phrase is by far the most commonest type of postmodification in English..")
4. A very nice girl from Las Vegas has posted this questions. Pay close attention now :
subject have still the same structure as in 3. - pre-modifier+ noun + post-modifier, but this pre-modifier is now complex i.e it is not a single word (nice) but a phrase. And that phrase is by its form adjectival phrase - very nice, and this "very" is an adverb by its form which functions as a pre-modifier of the adjective in the adjectival phrase "very nice".It intensify "nice" and it is specifically called intensifier,and I'm sure you could guess it without me telling you that.It's in its essence quite logical,just follow logic and you'll arrive at the right place.
To conclude,adverbial is a function in the sentence structure and a modifier is a function in the phrase structure (adjectival,adverb or noun phrase only).This is where you're confused.For example,you've stated "for four weeks"to be an adverbial in 2.I think it is correct,and you're correct that it is a prepositional phrase also,but you're wrong that this phrase is a modifier,since it doesn't modify noun,adverb,or adjective in some phrase.It serves its function on the level of sentence by giving some circumstantial-additional information on the time regarding the meaning of the whole sentence.Similarly, in 7." on his front porch" is an adverbial and is a prepositional phrase in its form but it is not a modifier for the same reason.It doesn't modify anything,but gives additional information-about the place,and add up to the whole meaning of the sentence.
And those were easier examples and as to the analysis of this sentences,I find them pretty tough for me to analyse and I'll have to think deeply.I mean deeeplyyy.And to read a lot certainly.I would prefer that somebody more competent do it instead.
But what is important about all this Cooklava is that you will make progress in grammar by reasoning on the matter like you do in your post.Way to go Cooklava!