OP, I too have had times that I've suffered with my mental health and been very tearful and stressed over small issues. I know this is worrying you, but honestly, on the scheme of things it isn't that big of a deal (to the council).
Just wait and see what happens with the council - I strongly doubt nothing. I'm guessing the rules regarding soak away drainage are for new houses and estates. Think about all the people who've replaced all their grass and shrubbery in the back garden with paving slabs!
If it will make you feel better, put aside a little bit of money each month to cover the cost of returning it to block paving. If the council do send you a letter they'll just tell you to charge it back, no drama, some expense, but not the end of the world. It's not like you built a house and hid it behind baked of hay to deceive the council!
But... How will they know you've changed it? Just hold fire and wait and see if anything happens.
Focus on looking after yourself, it's a shitty time for everyone, and if you were already struggling with your mental health then this last year is going through exacerbate that. Take every day as it comes, and try and not to worry about things that haven't happened.
(P.S. I am a worrier and always panicking about what ifs too. Sometimes you need to tell yourself to sit down and look at the bigger picture.)