I know Stones has played at RB a few times this season, but more than CB? Sounds like a made up "fact" that given Colemans involvement in the majority of matches. He's simply not a RB, he's a CB by trade & should play there; Sterling beat him easily numerous times when he was at RB against us & should have cost his side a penalty.
I fail to see what qualities Mangala gives at LB, he struggles to pass & can't say I've ever seen him attempt to cross a ball.. You say he can play LB, but when did he last do so? Not researched this but far as I'm aware he was out & out CB at Porto. (He used to be utilised as a Striker early in his career, that certainly doesn't mean he can play there though).
A key area of our attacking play for the last few years has been Zabaleta & Kolarov/Clichy bombing down the wings offering themselves to get crosses in the box. If you think Stones/Mangala as fullbacks will have anything to offer in that department I'd be surprised.
Again what is your point about the number of games? We currently have 4 CB's, with the same personnel you've mentioned, except we have MDM who is 35 & Argentinian so also has these "travelling miles". Next season we will have the same original 3 & Denayer, who's 20 & not even a starter for Belgium so will be far fresher. Are we suddenly expecting a significant increase in number of games next season? Because it seems an invalid point, in fact as I've shown we'd be likely to be better off next season even with no additions at CB in that aspect.
1 addition to CB could be valid if we're playing 3 at the back next season or if we offload 2 (MDM + Mangala); otherwise stocking up on 6 CB's is a huge waste of money & will hugely unbalance the squad defensively.