Well maybe is complicated to accept it when price goes down, and spending it maybe not affect the price when buy something with but when the shops sell it for money then there can be a drop, but if we will have a higher price i think it can be adopted and accepted because maybe then the price will not have so much fluctuation. And if shops sell it people can buy it again and use again.
if the payment system is as you explained, using a third party, then the third party exchanges it directly to fiat. this is tantamount to selling bitcoin to buy things right? if the user of that payment system (third party) is large it will certainly affect the market? I can't determine this is good or bad for the price, because of course there are positive and negative influences. I think it would be much better if we spend bitcoin directly to the merchant without a third party, it would be good for the price