How are the 24 character names determined?
I think it's an internal iPhone file name, so unintelligible to humans. I'm only guessing 24 - life's too short to count them
Do you have ability to rename as when importing?
They appear in your catalogue with the full 24 characters. I just rename them later.
Also, ca folders be defined as on import, such as 'iPhone/YYYY/MM'?
This is done quite well IMO. The choices are the same as the "date format" box in the regular import dialog box, so YYYYMMDD etc. I have both my phone and DSLR photos going into the same YYYYMMDD folders, but you can have different settings for regular import and for syncing. If you can set up your "iPhone...' in the Import dialog, you should be able to set it up in the mobile preference.
These folders also sync any images/video you may have created using other Adobe iOS apps (eg Photoshop Mix or Fix) and chose Save to Lightroom, or images/video which you have uploaded on another computer to your Lightroom on the Web page (lightroom.adobe.com). So today I exported a short movie from my laptop and dropped it into my "Current Work" collection in my web browser. When I get home it'll automatically appear in my main catalogue in the 20160328 folder.