Had a bit more of a think about this, when metaphor is described as a literary technique, it is generally used in the singular in my experience. As in, metaphor is the technique, metaphors are the result of using that technique. Same goes for rhetoric.

So when describing a technique the singular should be used, when describing the result of using the technique the plural should be used. So in the first example ('the deliberate use of metaphors, emotive language, rhetorical questions and punctuation...'), metaphor and rhetoric could be used either way and remain contextually correct. However, emotive language and punctuation are blanket terms denoting technique so I would aim for consistency and thus use 'metaphor' and 'rhetoric' as singular.

In the second example, 'metaphors are the most significant literary technique used by Lawrence', it can go either way. But as you are referring to a technique it should be singular. In all of the references I have found metaphor as described as a technique is always used in the singular.