First, let us clarify the basic rules:
When an article is presented at a conference, it is generally not complete. The general practice is to present a preliminary analysis at the conference and reserve the detailed analysis for a full paper. The purpose of presenting your research at a conference is to inform people about your study and get feedback from them to improve your methodology. So, it is okay to develop a previously published conference paper into a full-length manuscript and publish it in a journal later on. This is an acceptable practice, provided you clearly disclose the conference paper and include at least 30% new material in the journal paper.
However, if you publish an article in a journal, you should not present it at a conference, because the audiences already know about the entire study.
Now let us come to your questions:
1. While it is okay to develop a previously published conference paper and submit it to a journal, it is not okay to submit an article to a journal before it is presented at a conference. This could be considered as simultaneous submission, as you are submitting to both the conference and the journal at around the same time. Both the journal and the conference would very likely have explicit rules forbidding simultaneous submission. Conference dates are public information and published journal articles usually have both submission and acceptance dates. So, even if the paper is accepted after October, the submission date will reveal that you submitted the manuscript to the journal before it was presented at the conference.
2. You can make a presentation at the Society, publish it as conference proceedings, then develop it into a longer article (improving the study based on the inputs you receive at the conference) and submit it to a journal. This is perfectly acceptable. However, you should mention in your cover letter that the article is based on a conference presentation and provide a link to the conference paper.
3. You cannot submit to the journal first and then submit the abstract to the Society.
The best course of action in your case would be to submit the abstract to the Society now and immediately start working on the full-length paper for the journal, so that just in case your paper does not get accepted for the conference, you can immediately submit it to the journal. In case, the conference paper is accepted, you should wait till after the presentation, make the required improvements based on the feedback you receive at the conference, and then submit to the journal.