In my opinion ADS does not apply as you are replacing your main residence and the following guidance from the Asvotyish governemet says it doesn't matter if your main residence is bought or rented. See page 27: https://www.revenue.scot/sites/default/files/LBTT%20Additional%20Dwelling%20Supplement%20Legislative%20Guidance%20-%2016%20March%202016_0.pdf#page27.
I think you should challenge this. The law is contradictory on this and manifestly unjust if interpreted according to Revenie Scotland as you will appreciate. There is no case law on this yet as far as I am aware and I would take it to a tribunal if I was in your position. It is a nonsense that people who own a house as their home and buy a new home are exempt if they have a rental property but someone who rents a main home but them decides to buy one is landed with the ADS because they too have a rental property. That was not what the legislation intended.
LBTT is a self assessment tax. For that reason I would have your solicitor put the return through as "ADS not applicable" and leave Revenue Scotland to challenge it if they see fit to do so. If they make a formal assessment that it does apply then you can appeal.