Bridal shower?

Showers are events that are hosted by others in your honor since the purpose of the shower is to "shower" the bride in gifts. If you host your own, it looks pretty gift-grabby. If someone else (your parents maybe since you mentioned their house) has offered to host the shower for you, then it's fine to accept their offer. I don't know if I'd center a shower around getting in the pool as that seems more like a pool party than a bridal shower, but having the party in the backyard is a fine idea. I'd leave the planning of that up to whoever offered to host. The only thing you should be taking care of for the shower is the guest list, which leads to your last question. The only people who should be invited to any pre-wedding events (including your bridal shower) are people who are invited to the wedding. Again, if you invite others who end up not being invited to the wedding, it comes off as very gift-grabby. The only exception to this is if your coworkers throw you a work shower. I'd simply stick to the people whom you know are definitely on the wedding invitation list for your shower.

You don't throw yourself a shower, someone else does. It's rude to throw a gift giving event for yourself. If no one else offers, you don't have one. I think the end of July is a little early for a shower for an October wedding, but that's a matter of opinion. The host(s) of the party determine the menu, but it depends on the formality of the event. If they decide to do backyard BBQ style, hot dogs would be fine. It's also extremely rude to invite someone to your shower who isn't invited to the wedding.

Unfortunately, I think this is going to have to be an either invite all or invite none situation. Maybe you could have a lunch/dinner with those girls after the wedding to celebrate with them.

I personally wouldn't bring it up the shower to them if they aren't going to be invited to the wedding. I'd more than likely say something along the lines of, "We unfortunately aren't able to have everyone we'd love to have at the wedding due to budget constraints, but I'd love to celebrate with you after." Then I'd probably plan a lunch/dinner in which I covered the expenses for all of them (could even be a low-key BBQ) so they didn't feel like they were being invited out only to foot their own bill.

It should be organized by your friends. You should invite them, its not much of a gap.

Oh course you should allow your friends to throw you a shower and the back yard is fine doesnt need to be big thing but only invite people who are invited to wedding.