General :: Activity Log Database - Combo Box Queries

I've attached my database as I have it now. I am trying to create what amounts to an activity log.

I have created my main Subject and underlying related Issues tables, and a table for my daily activity notes. I have created my main form and subforms that should be sufficient (hopefully) for my purpose. Where I am getting hung up now is making the Subject and Issues fields combo boxes and filling them.

For the top-level Subject, I want to be able to type a new one in at any time, but also have the option to choose from a dropdown on the control as well. Then, if I do type one in, the next time I try to add a new record, it would appear in the dropdown as well. I think I have to query the underlying field to drive the dropdown list, but I cannot remember how to do that.

For the second-level Issue, there would also be a dropdown in the same way as the Subject control, and I want to be able to choose from the dropdown or add a new one in the same way... but I need to make sure that the ones that appear in the dropdown are related to the top-level Subject shown in the main part of the form. Of course, I don't want to be able to add an Issue unless it is related to a parent Subject. On these things, I am totally lost.

Finally, I built my notes to show in the subform as a continuous form, which I have used only once before. I'd like it so that the notes show newest at the top. And, I'm thinking about putting a "lock" checkbox on the note record so that I don't overwrite previous notes unless I consciously go back to edit something. I'm not sure how I can make an individual note dependent on an individual checkbox in the continuous form.