Seeing that Miami plays a lot of 3 receiver sets, I think you would be playing with fire if you only kept 4. One injury and you either have to change your offense or lean on the remaining 3 all the time with no rest and pray nobody else gets injured. I think you need more than one back up receiver.
Also Smith seems pretty slow to be returning kickoffs. I would put Williams back there first.
As for the punt return duties, Drake is trying to learn how to do it, but we saw how that went for Grant last season. It's hard to expect a player who has never fielded punts before to do it at an NFL level year one. I personally am leaning toward the thought that if you have as much depth at receiver as you say you do, and you believe that Parker and Stills can carry the load why not let Landry handle the job. In the past you didn't want to risk Landry to injury, partly because we didn't have much else at receiver. And seeing that Landry might lose a few targets and receptions as the team tries to increase the roles of Parker, Stills, Drake, Thomas and Ajayi why not give him a few touches on punt returns. Sure there's a risk of Landry getting injured, but as hard as he plays there's a risk every time he's on the field. Replacing 4 or 5 offensive snaps with 4 or 5 punt returns might not be much more risky.