Woo.
On inference: there are a couple of instruments that might give a better look at inferential comprehension. If appropriate error analysis is done, even the WIAT-4 and KTEA-3 (standard academic achievement tests) can tell you a bit about inferential comprehension vs literal comprehension, or comprehension of literary vs informational text. Assessments that are typically given by speech and language pathologists also can tease a bit of this out. The CASL-2 supralinguistic index includes subtests of inferential thinking (and other metalinguistics). Comparing the core receptive/expressive language portions of the test to the supralinguistic portion can highlight strengths/weaknesses. There's also the SLDT (Social Language Development Test-- it comes in an elementary version, which ends with age 11, and an adolescent version, which starts at age 12; if they go this route, make sure the correct version is administered).
And on the multiple choice vs open-response: I would look into more extensive memory and learning testing, such as with the CMS or (preferably) WRAML-3. That can identify differences in aspects of memory, such as between free and cued recall, and short-term and mid/long-term memory.
Writing inefficiencies: something like the PAL-2 can give you a sense of where writing breaks down: physical, speed, formulation. Some aspects are also on the new WIAT-4, if all or nearly all of the writing tasks, including supplementals, are given. The TOWL-4 continues to be the gold standard for writing assessments, perhaps with the addition of one of the timed sentence writing fluency measures from the WIAT-4 or WJIV. There are hand copying tests (should be part of the OT or AT evals) for word/sentence copying speed and legibility. (E.g., the Wold.) I also like qualitative assessments comparing how he does with a writing prompt under three conditions: handwritten, typed, and scribed.
