I’m sorry you’re finding it frustrating - I’ve dedicated the last seven years of my life to Scrivener, and to making it as intuitive as possible, but of course not all software will suit everyone. Compile can be a little complicated simply because it has to be flexible enough to allow for any structure in the binder.
That said, you seem to be asking about something different, and I’m not entirely sure what you are trying to do.
I think the first thing to point out - as I feel that this may be the source of confusion - is that Scrivener does have a “styles” system; it seems as though you are expecting presets to work like a styles system, but I may be wrong.
Next, there is absolutely no need to edit the “Text Preferences” in the Project menu unless you want to use a different format when writing in this project than in other projects. So, if you are just trying to set a default font and paragraph formatting for creating new documents in Scrivener for all projects, forget these settings (and deselect them).
I’m not quite sure why you have set it to use a bold, centred font in your screenshot; from your description, it sounds as though you have applied the “Title” preset to this formatting, but again, I’m not sure why you would want to do that. (The “Presets” are just a set of preset formatting that you can choose to apply to text anywhere in Scrivener. The text in the “Main Text Style” box is sample text that shows you the formatting that will will be used when you start typing in an empty document.)
Moving on to “Main Text Style” in the Preferences. This sets the default font and formatting that will be used in new projects.
So, all you need to do is ensure that the formatting in “Main Text Style” in the main preferences is as you want it for new documents - it’s as simple as that, so I think you are making things more complicated than they are.
Moving back to “Presets”. You can edit these and change them to be anything you want. (“Title” is set to be 28-point bold using the current font; “Body” is 12-point, left-aligned, using the current font. But you can edit them by typing some text in the format you want and going to Format > Formatting > Redefine Preset…")
Now, when you use Convert > Formatting to Default Text Style, all of the text in that document will be converted to use the formatting you have set in the “Main Text Style” of the Preferences. There is a warning that pops up telling you about this.
As for having a section title appear on the same line as the body text in Compile, click on “Level Settings” in the Formatting pane and tick “Insert title as run-in head” under the “Title Appearance” pane.
I hope this helps.
All the best,