I’m not sure if I’m missing something here, but the type of style you want to create is possible in Pages. (The heading style is a different story though.)
You can create a new list style that uses a combination of numbers and letters.
The lists below are from a list style I made in pages.
I suggest you create the entire style from the start with all the levels you’ll be using. Things can get a bit confusing if you do each level on its own at different times or you start with levels in-between. The process might seem a bit complicated, but it’s actually easier than it seems once you understand the process. Anyway, I’ll walk you through the steps.
First, type a word (‘car’, in my example) to create the top level (A).
(You need to type in something, even a single letter, in order to be able to set up the entire list style properly).
Then go to the bullets & lists settings. Select the Lettered list style from the pop-up menu (where it says ‘None’). Set the indents for number and text as needed; click the same pop-up menu one more time and the + sign (top right corner) to create a new style, and give it a name (e.g. Letters & Numbers’).
Then press Return to go to the next line in your document. Press the tab key once to start creating the next level down (A.1). You get ‘A.’ again. That’s OK. Type another word there (‘train’ in my example) and go over to the bullets & lists panels to activate the tiered option. (You get ‘A.A.' now). From the drop down menu (‘A. B. C. D.’ — above the ‘tiered’ option) choose the numbers (1. 2. 3....) for your tiered style —now you get ‘A.1.’. Also, set the indent for the number to ‘0’ (since you want all levels to be flush-left from what I saw in your post), and set the number for the text indent to whatever works best for your use. Now you need to save the settings to the 2nd level. Again, go back to the Bullets & Lists pop-up; there you’ll see a blue ‘update’ button next to the name of your new style (‘Letters & Numbers’ for mine). Click the ‘update’ button to add the new level-2 settings to your style. Now you have both level 1 and level 2 set up within your style.
Repeat for the third level: Hit return to move to the next line. Type in a word—’ship’ in my example. Press the tab key to go down a level (A.1.1); the options for ‘tiered’ as well as the number style should carry over from the level above; if not, repeat as above. Set the indent for number and text again. Go back up to the Bullets & Lists pop-up and click the ‘update’ button again to include the new level-3 settings in your style. The third level has been set up as well.
If you think you’ll need a 4th level (A.1.1.1.), repeat the steps above one more time. If you aren’t sure, perhaps creating one or two more levels wouldn’t hurt—just in case.
You’re all set. To assign a particular level, go to the Bullets & Lists pop-up, select the style you have just created, and then use tab key on your keyboard to go down a level (from A.1 to A.1.1 etc), or shift+tab to go up a level (from A.1.1 to A.1 etc) or the dedicated buttons below the text color settings. So, if you want to use A.1.1. for instance, hit the tab key twice after you have assigned the list style.
It must have been quite tiring reading all this, but if you follow the steps, you’ll get how it works.
Most importantly, I hope it's what you are trying to do.
