I've distilled my previous objections to Evangelicalism down to a 16-word statement; I don't know how much clearer I can be:
The grammatico-historical method is neither a necessary nor a sufficient condition for objective meaning of Scripture.
This is the super-concentrated basis for the statement.
-- Existing cases of text serving as objective social norm when communicative intent is inscrutable or no information is available (e.g., law); GHM not used.
-- (Historical) Unknown in early church, but theology still formed without and often contrary to results of GHM
-- (Historical) Church acted like a society with bishops as judicial interpreters would be expected to act (viz., treated Scripture as objective social norm rather than communication)
-- Even within GHM, people operate based on speculative reasons as if they are sure despite lack of consensus (viz., in practice, certainty under GHM not necessary even in communities that practice it).
-- Contra a far too common argument, not necessary to resist postmodernism (metaphysical realists can deny postmodernism without difficulty).
-- Justification for GHM undercut because:
* Assumptions that give GHM empirical reliability not met or contradicted in case of divine authorship.
* Even if it were demonstrated that the GHM were necessary for objective meaning, this would not demonstrate that the GHM was sufficient for objective meaning.
* Sufficiency of GHM is entirely contingent on circumstances of historical record (viz., it is not prima facie sufficient for any particular application; it can be sufficient in one case and not others).
* Lack of metaphysical safeguards in method leads to philosophically untenable results
* Lack of demonstrable consensus on detailed standards of the method
* Lack of demonstrable consensus in certainty of outcome of application of the method (high subjectivity)
* Analogical reasoning to apply Scriptural norms to present day entirely subjective
* (Scriptural) Pharisees likely had access to tools of GHM; Jesus and Apostles offered counterinterpretation based on other Christ-centered interpretive techniques (e.g., peshar) and considered Pharisees blameworthy for not following them.
* (Historical) Rejected outcomes of historical application of GHM-like methods (e.g., Antiochene exegesis) in speculative theology. Affirmed outcomes of Christ-centered allegorical methods (viz., interpreting passages in Christological terms, irrespective of original intent).
Those are my reasons for doubt in a nutshell. If some Evangelical can come up with an argument why the GHM is both necessary and sufficient for Scripture to have an objective meaning, more power to them. Note that I am NOT being inconsistent in my application of the GHM to some cases and not others, because: (1) the inapplicability of the assumptions of the GHM in this instance render its application dubious, and (2) the sufficiency of the GHM for any purpose is case-sensitive (viz., it is always debatable whether the GHM will work in any particular instance even if it works in other instances), putting the burden of proof on sufficiency on the one asserting its sufficiency in that case rather than vice versa.