French language Q&A Forum

I have two separate questions regarding the same example 

The first is…why do you use “sa tête” to mean “his face”? 

The second is…I am sure that there are many colloquial ways or common ways to express “should have” using the verb avoir in its conditional conjugated form when it’s not followed by “dû”, but is it actually proper French grammar? For instance, in one of the above examples, it reads: “Tu aurais vu sa tête quand je suis apparu devant lui.”  Its translation is “You should have seen his face when I appeared in front of him.” Possibly contextually it translates better to should than would. Perhaps if the sentence was “ Tu aurais vu sa tête, si tu avais été là.” Then it’s a true conditional statement-You would have seen his face (condition) if you had been there. So perhaps I’ve answered my question because this really isn’t a conditional statement However, I like rules, I like things to follow those rules (The Container Store is one my happy place-quote from Emily in Paris). I also realize that as I write this, the English language is known for not always following grammatical rules in one sense or another (although I can’t think of any because it makes sense to me as a native English speaker, so please forgive my hypocrisy). Please help me understand when avoir in its conditional form means should when not followed by dû.

Why is my answer (deux milliards d') wrong? The lesson doesn't explain plural milliards.

Elsa has two thousand euros on her account.

Correct: mille
Incorrect: milliards d'

I always have difficulty deciding whether it should be 'leur' or 'leurs' in these circumstances. I opted for 'leurs' this time and it was marked right - both ar accepted here! But thinking about the logic, it seems to me that it should have been 'leur': There are lots of friends, but each of them just has one family - so 'leur'.

Or am I barking entirely up the wrong tree here ?

In the example, all the indirect object phrases start with à or au.   Au marché, à Paris. In the quiz, my answer got marked wrong.  Il va à chez Jean.  The correct answer appears to be il va chez Jean. Is ‘chez x’ a special case that does not require à?