I hope this message finds you well, present circumstances excluded. I am a licensed attorney with over a decade of experience handling matters of this nature. It is a pleasure to assist you today.
First off, I am sorry you are having to deal with this issue. Second, and to the point of your question, my recommendation is that you seek remediation for this incident with the Walmart Corporate Office in Bentonville, Arkansas. You contact them by phone first and follow up with an email as to your treatment and the way this treatment made you feel. You need to explain that you feel as though you were discriminated against based on your disability. Explain that you have spoken with counsel (which you have now) and that you know your rights. Explain that you are aware that you can file a complaint with the US Justice Department and subsequently file a federal law suit. Tell them you don't want to do this, but feel as though their treatment of you (and likely others) may warrant such an endeavor.
Ask them to make this right (and explain that a $25 gift card is insulting relative to the embarrassment and treatment you received). Tell them to check with the store, as they should have documented this (even though they claimed that they had no complaint forms).
Follow up with the person you spoke with via email. Give them 10 business days to make a decision. Tell them if they do not, then you will likely pursue legal action.
If they do not do better for you, I think you need to file a complaint based on violation of your ADA rights with the US Justice Department.
Let me know if you have any other questions or comments.
Please rate my answer positively (three or more stars) so I can receive credit for my response.
Best wishes going forward!