Late reply because I went to sleep last night.
You are definitely extrapolating on the dialogue. Snoke does not say he is terrified of Luke. They do not say, "We must go to Plan B and blow up the Senate to keep Leia from Luke" That's your interpretation of what is otherwise straight dialogue describing the multi-pronged plan of The First Order to take over the Galaxy. The attack on the Village was to find the information leading to Luke, yes. And as a result of that Attack and the subsequent open attack on Jakku during the hunt for BB8, they decide to take out The Senate before they can get involved because The First Order has technically broken it's treaty. The ultimate goal is not "cuz OMG if The Republic gets involved they'll bring the mighty Luke!!!!1one" it's because if The Republic gets involved it will be a full fledged War with an equally equipped enemy. Yes that enemy would unite with The Resistance but even The Resistance's goals are more than just Finding Luke.
What you're missing is that while the film itself centers around the aspect of finding Luke, that's not the only goal of any of the major parties involved. The Resistance ultimately wants to defeat The First Order and The First Order ultimately wants to take over The Galaxy. So while a decent chunk of what we see is steps taken to find Luke, it's not the only goal of either side and with Snoke and The First Order specifically we see actions taken that move forward other aspects of the overall plan. Destroying The Senate and The Republics armada prevents The Republic from mounting an offense against them in the future with or without Luke. The Resistance destroying Starkiller Base removes a genocidal superweapon from The First Orders arsenal, again, with or without Luke a goal is being accomplished.
As an audience, we are primarily focused on the Skywalker family. We know their overall importance in this Universe but that does not mean that the entire Galaxy wide conflict is actually centered around them.
Had Snoke been anywhere near as fearful of Luke as you claim, he wouldn't have trusted only two commanders (Hux and Luke) to deal with the problem no matter what they said. He would've brought the entire force of The First Order down upon Jakku OR Takodana and eliminated any and all chance of that information escaping his grasp. He doesn't do this because there are other concerns. Other things throughout the Galaxy that have his attention. Luke is a major priority but not the only priority and not everything he commands is about getting to Luke.
I'm extrapolating nothing.
Here's how the first exposition chamber scene goes.
1) Just before BB-8 escapes with Rey and co on the Falcon
2) Chamber scene: Hux or Ren says the android escaped and is surely on its way to the Resistance.
3)Snoke says then we must change our strategy and says something about how the new Jedi must not be allowed to rise
4) Hux says the weapon is ready. Snoke says good. Hux says by taking out the Republic Leia will have no allies and thst they will easily stop her before thry find Luke
5) Hux leaves and Snoke and Ren talk about feeling the force awakening.
6) Nazi speech and Republic Senate goes boom
Next chamber scene
1) Rey is captured, Ren orders them not to worry about BB-8 anymore
2) Rey escapes
3) Second chamber scene
4) Ren tells Snoke she escaped.
5)Hux says Ren ordered them not to get the froid because the girl was supposed to be enough so now surely the droid is in the hands of the Resistance.
6) Snoke orders they target Leia's base with the weapon and destroy them before they can get to Skywalker.
7) Rest of movie happens
This is what happened in this movie. I didn't extrapolate anything. Snoke considers preventing the Resistance from finding Luke to be their top priority and every decision he makes in this movie itself is with that goal in mind. This is explicitly stared at least twice in frankly heavy handed exposition. I don't understand how you missed it.
Had they got that droid in Jakku they would not have blown up the Senate.
