Straightforward answer: Yes, as Pablo Hidalgo of the Lucasfilm Story Group recently said here and here, it's a mistake and that the lightsaber is still supposed to activate with the plate in the middle.
Q.Why in the Force Awakens does Finn clearly press the blade adjustment button on Luke's lightsaber to turn it on? Is it Snoke?
Let's say it has both the activation button and the length adjust control as depicted in the dictionaries. Presumably the activation button would cause the blade to quickly jump up to the set blade length. However, if you left it on, but turned the length to 0, maybe holding the length adjust button down would cause it to extend in a slow, cool, way! That way, both the OP images could make sense.
In episode 2 of the unfinished episodes of the Clone Wars, which I believe are canon, Anakin turns his lightsaber on by pushing the big red button in the front. This, added to the many ways you cited by which this lightsaber can be turned on, leads me to believe that there is in fact more than one way to turn it on.
I would say it works like a dimmer switch. One switch just controls off and on while the other can control blade length and intensity while also controlling the on off capabilities. Redundancies built into the weapon to make it more useful.
This also explains why Darth Vader comes out at different speeds. Sometimes he uses the blade length adjustment knob sometimes he uses the on off switch
I'm Star Wars fan almost for 20 years, and I remember that in old sources Younglings and Padawans has to HOLD the button if they want lightsaber to be on, and when they were ready, they adjust switch which can be blocked in ON mode (for example to throw saber).
BTW there are flashlights that have similar two switches, one to push and blink, the other to slide and be on still.
I think the answer is simple, you can turn the light saber on from two locations, the adjustment button and the activation matrix. You clearly see Fin start the lightsaber from the adjustment button and that movie is canon so that mean it can be started from there. You also see Luke start it from the activation matrix so you can start it from there too. It’s got 2 power buttons, simple.
calm down. Lightsabers are actually activated by pressure, not a switch. The people who activated it before could have just pressed it on a different area. Besides this movie is forty years old it would make sense that they could've gotten it mixed up in production.
Anakin probably wired multiple activation buttons just in case he turns it off mid battle. Then he could use a button in a different location to make sure it stays on. There could be many different buttons on anyone's lightsaber.
Jedis tend to make their own light sabers and can fiddle around with button configurations howsoever they wish. Ask anyone who has programmed a macro for playing a MOBA or MMORPG and they'll tell you the power of custom mapping.
I noticed that right away. The way Rey and Finn turn it on, seemed to me just a stupid mistake. Obviously the young actors grew up with silly cartoon versions and prequel of SW (where is not clear which button they switch, since it changes all the time), they do not know where the actual switch on button was in the original trilogy (where Luke actually turns on the lightsaber always in the same rectangular spot in the middle of the handle, and the replica has the exact same design), and apparently no one, either in pre-production, or on set of Ep.VII TFA, bothered to check for continuity purpose, not even the director! Lame...