Legislative drafting ought to be the pinnacle of legal endeavor, but sadly this is not the case. Indeed, a glance at the House's legislative drafting guidelines show that assessment of constitutionality is barely discussed, and then only insofar as it pertains to the mechanics of passage.
While the constitutionality of a proposed bill is properly a matter for debate on the floor of the legislative chamber, in practice a great deal of debate seems to consist of procedural maneuvers at the expense of substantive discussion - and as you say, the substantive changes wrought by a bill are often couched in the form of obscure and minor-seeming tweaks to existing legislation that can result in dramatic changes of scope or authority despite their superficial blandness.
http://legcounsel.house.gov/HOLC/Drafting_Legislation/Drafti...
While the constitutionality of a proposed bill is properly a matter for debate on the floor of the legislative chamber, in practice a great deal of debate seems to consist of procedural maneuvers at the expense of substantive discussion - and as you say, the substantive changes wrought by a bill are often couched in the form of obscure and minor-seeming tweaks to existing legislation that can result in dramatic changes of scope or authority despite their superficial blandness.