If you think you are in your right then you should email MGA.
Usually every casino checks or you make the deposit with a card thats on your name. Sometimes they allow it if you can make a selfie with you and the card you are using.
On the other hand, it is against the TOC to use someone else his card, so if you are doing that ALWAYS contact the casino before your making the deposit.
About the fact you have been losing and that is all fine, thats just how casinos work. They dont check every account how they are making their deposits. They wont see your account till you actually ask for a withdraw.
My advice to you now is to contact MGA and explain the situation. You said you had contact with the live support from SlottyVegas that they were okay with it. I believe SlottyVegas emails a log to your email. Add that aswell that they approved with it and that you are willing to make a selfie with the card you have been using.
I think the biggest problem here is that you used someone else his card without black on white a confirmation for you to use it. So it could be a stolen card. Keep that in mind aswell while typing to MGA.
I wish you best of luck and hopefully I made it a bit clear.