That's correct (mostly). Sa (and "ma" and "ta") are used with feminine objects in the singular as long as the word doesn't begin with a vowel.
ma robe, ta pomme, sa salade
Words that begin with a vowel get "mon", "ton", or "son". Plurals get "ses" irrespective of the gender of the object (mes/tes/ses pommes).
(the "nous" and "vous" possessives only change between singular and plural, and not with regard to gender)
you are correct. Also I would like to help you- that was very good adlib! pronunciation wise 'aider s'il vous plaît' is correct. but, what you need here is probably a tense you haven't learned yet. or maybe just a mode? anyway, just so you can see it, what you are actually saying is 'aidez, s'il vous plaît', because you are asking (you) help please, where the subject is inherent in the interocative form (ps... that's the form you're using ;) ) I don't mean to be rude, just want to help.
Apparently, there are is no present progressive tense in French. If you want to emphasize that the activity is in progress, you may want to use "etre en train de". In your case, it'll be "Vous etes en train de mange son pomme" which literally means "You are in the process of eating her apple". See http://french.about.com/od/mistakes/a/jesuislisant.htm :)
"sa" is feminine, "son" masculine. However it does not matter in this case (and to answer the second part of your question); you need to agree with the gender of the item that is owned, which in this case is an apple. Apple is feminine so it's always is "sa pomme" regardless of the gender of the owner.
I couldn't wrap my head around it either so I googled it and got this table clarified it all for me: http://www.frenchtutorial.com/en/learn-french/possessive/adjectives
One of the best websites for learning French rules (that i have found anyway) is www.laits.utexas.edu/fi/ Sounds like a wierd web address for learning French but it breaks down the 'rules' of the language in a way thats very easy to understand. Its helped me so much with completing levels on Duolingo.
The first part of the word tells you whose it is:
1st person singular -- mon, ma, mes
2nd person singular -- ton, ta, tes
3rd person singular -- son, sa, ses
1st person plural -- notre, nos
2nd person plural -- votre, vos
3rd person plural -- leur, leurs
The last part of the word tells you if the thing possessed (not the person who possesses it) is masculine or feminine or plural:
Masculine singular -- mon, ton, son
Feminine singular -- ma, ta, sa
Singular, masculine or feminine -- notre, votre, leur
Plural, masculine or feminine -- mes, tes, ses, nos, vos, leurs
Yeah, I did read the notes and tips before I came in, but I'm either forgetting them or mishearing things. This unit is far more complicated than any previous ones. Sentences that could just as easily say 'les' actually meaning 'ses'. And yes, I can remember that when the sentence comes round again but if I'm not getting it right first time on some childlike learning app, I'm not going to be able to understand it in actual France.