I can't answer the topic title, unfortunately, I'm just not sure, but I can tell you with certainty, that regardless of how much +rng you might happen to have or your henchmen happen to have, it will not prevent them from running into melee combat very frequently.
The AI is borked in this regard and nothing short of marshalling your henchmen every time they decide to run into combat will prevent them from doing so. Even then, they may decide to ignore the follow command completely until they've executed whatever power (even if it's a ranged attack) they seem wholly determined to execute from melee range. Then they might decide to follow you again. Maybe.
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To the best of my knowledge, none of the pet summon powers accept range enhancements, so they shouldn't be affected by it. Plus what Zbron said about the bad AI.
As far as I know, only Centrioles will give pets extra range because of the way HOs are passed down through pet summons. Cardiac shouldn't work because using that ability is just like slotting a range SO into a pet summon, and that alone will not grant more range on pet's attacks. Any buff power such as Clarion that grants +range will work too. Some of the demon's ranged attacks are flagged to ignore range enhancements though.
Last time I checked, Centrioles worked in Mercs and Thugs. For kicks, get a second build and try slotting 3 Centrioles in each of your pets and get the Clarion with the +range buff. Your pets will mostly likely have sniper range on all of their ranged attacks for the duration of Clarion. I guarantee that the stupid AI will make them run into melee range however...
Does it? I'm merc/TA and I'm pretty sure the only way my pets are going to survive is to never see a meleer upclose again. And since 95+% of my attacks are ranged it wouldn't really hurt to stay there. Especially w/ the tiny cone powers.
Edit: Would the clarion +rng do anything if cardiac doesn't?