two things to point out
1. verizon will have no problem bringing in your iPhones. the 6 and 6 plus should be accepted by them. the 6s and 6s plus probably too. go to verizon's byod imei checker to verify your phone is compatible
http://www.verizonwireless.com/bring-your-own-device/
2) verizon does not have a whole lot of MVNOs. i think the only Verizon MVNO i ever heard of was Page plus. i think they are others.
the posts you are reading are very old. in august 2015 Verizon flipped the switch and allowed every unlocked iPhone 6 to be used on their network. they did the same thing to google nexus phones . they don't care anymore, as long as it has cdma+gsm
i think the way the 6s and 6s+ now is, if you have the AT&T model ( a1633) you can use it on verizon. but if you have the verizon model (a1688) AT&T won't activate it. you have to take that one to cricket
the way the 6 worked was that T-mo , Verizon at AT&T used the same phone, and it was just sprint that wanted their 6
now its AT&T that wants their model
other then that. the only other thing to add is when apple releases a new iPhone on the market. it can take individual phone carriers 60 to 90 days to put the IMEI in their database of accepted phones. you need a good IMEI to activate the sim card. when i bought my 6+ and 6s+ both times i had to wait until October/November for some phone carrier to say my imei is ok, so this is another good example on how the posts you are reading are old..it looks to me there is an 'exclusive window' that phone companies will only activate phones in their books