Sounds good so far.
To address some points:
I am not Christian, but gospel music for a smal portion of the party would in no way offend me (heck I wouldn't mind if it was the only music). I can't imagine anyone would be offended unless it was a straight up church service ambush or unless someone was actively trying to convert them.
I think your mum is wise to plan around her best friend's schedule. Throwing a party, while fun, is stressfull, she should have her close friend around. If people are starting school early Sept, a later in Sept date might be better anyway, as they will by then be a bit more settled. many people see it as bad luck to celebrate before the big day anyway (regardless of occasion).
Office supply stores (in the US, for example Staples) usually carry really, really nice invitation kits for very good prices. They are formal style invites, with envelopes, response cards and response envelopes. Usually you can buy matching thank you or note cards separately. The kit comes with templates and sample wording and everything easily fits into a home computer printer.
A nice thing for you to do, if you are doing invites or especially RSVP's is to ask people to bring (or if possible send in advance) a card or piece of paper (you might want to provide so it's uniform) with a memory, favorite story, whatever involving your mom, which you can then incorporate into an album for her (perhaps combining with photos from the party, or even old photos of you mom with the people who provide a memory).
You might want to prepare a speech, and possibly have some other key people in your mom's life prepare ones as well. I often have seen/heard speeches at birthday parties of this nature, and they are usually quite touching or amusing. And they lend an air of formaility to the event.