It looks like my understanding was correct. You created a custom question using shapes. Maybe you can have a look at this recent video I posted about a custom MCQ. My setup blog is waiting for moderation:

Custom Multiple Choice Question – eLearning (adobe.com)

I have some ideas in mind, again including shared actions because this is also a typical use case for them.

You can add a custom state to each shape, which has the right character added, and use the Success event of the shape button to switch to that custom state, replacing the Normal state. Alternative is to use an extra multistate object which has the characters for both correct and wrong answers. Normal state of that object should be empty. In that case you can use that unique multistate object for both wrong and correct answers. Reflecting on the best setup to make the shared actions easy to use takes some time but can allow you to save lot of time later on.

Disabling the shape buttons needs to happen with the action on the Submit button.  You cannot use grouping, which is a long delayed feature request to be able to disable a group of interactive objects. You could eventually do that with JS.

I suppose you talk about version 11.5? That is the most recent version.

Not sure to understand your question correctly. You talk about ‘shape button answers’? Gaanf seems to suppose that you are talking about question slides, but a shape button cannot be used on a question slide? Are you talking about shape buttons used on a custom question?

Anyway if the feedback messages are shapes (not captions) it is easy to add any object to the message itself in its normal state, as Gaanf explained.  Normally shapes are by default used for all feedback messages, except for those appearing in software simulations where the old captions are used. Check this in Preferences.

First part of the question – showing ‘Correct’ and ‘Incorrect’ characters along with the feedback message:
You don’t really need any advanced actions for that. Just select the ‘Correct’ feedback caption, go to ‘State View’ and add your character there. Same for the ‘Incorrect’ feedback caption.

Second part of the question –  prevent reselecting another answer choice:
Are you saying that with ‘No. of Attempts’ set to 1 on question level, a learner still can alter his/her choice AFTER having submitted the answer? That’s not expected behavior and that’s not how it works for me. Something must be off.
If you mean they can alter their choice BEFORE they clicked ‘Submit’, then that’s expected and the way it’s supposed to be (since they might have accidentally have clicked the wrong choice first, and should have a chance to correct that).
Maybe you have checked ‘Submit All’ in the click preferences? Then a learner could indeed change their selection on the current and previous questions, as long as they have not yet clicked the ‘Submit All’ button.