I prefer "becomes untethered." Reasonable speculation is closely based on the evidence (is closely tied to the evidence). Bold speculation moves farther from the evidence (but is still tied/tethered to the evidence). Pure speculation disregards the evidence (moves completely away from the evidence, so untied/untethered). "Becomes substituted" is understandable, but more natural is "substitutes." "speculation too easily substitutes for evidence."

I think number 5 is best from the choices you offer. "speculation too easily becomes substituted for evidence" Actually number 3 would also work. Why not the others - because they don't make sense in this context. How can speculation be constrained by the evidence in your context