Not necessarily disagreeing with you in my reply here.
Lage started a left-footed, left-winger, Pedro Neto, at RWB against us, a universally acknowledged stronger team, away, and he and they both had a pretty decent game against us. Why shouldn't Perisic be able to do the same thing (RWB) better than Neto, who barely has any previous experience at WB?
Conte also has switched to a 4-2-3-1 when we were chasing against Chelsea, so he has shown the willingness and flexibility to do exactly what you say. You are right though- it's doubtful he ever starts a back 4 while with us. We'd have to be decimated by injuries, suspensions, or otherwise for it to happen- he's clearly a man that likes a back 3.
I'd say our most obvious area of need right now is at RWB. Emerson just does not look dangerous when the opp sits back and dares him to hit a cross or make a pass. Hes not that great at taking his man on either, though he is a decent runner in the open field. Play dead-end'ed with him several times versus Wolves, which was of course disappointing. Any one goal victories are too close to consider a comfortable win; we need to be aiming to do better.
I'd like to think that Spence is the answer, but Conte can't start him soon enough so we can find out if he actually is the eventual answer within this window. So we're kind of in a pickle, having gambled, and in process of gambling on the future right now, not knowing if it will pan out. If we're looking for attacking balance and threat, I still think its worth trying Persisic at RWB at least until we can figure out if Spence is the answer.