The SBV and WISC-V are both individually-administered, which is quite a different proposition from group standardized tests like the CogAT. I've only had a few months experience with the WISC-V, as it just came out in October (though years of experience with the previous versions), but generally, I like the structure of it better, from a clinical utility standpoint. The SBV is much less dependent on speed than the WISC-V, but even on the WISC, only a handful of subtests are really affected by speed, and two of them are specifically measures of processing speed. (The SBV has no measures of processing speed, which can be a clinically important aspect of cognitive ability.) One of the improvements with the WISC-V over the WISC-IV is that speed and working memory are less heavily weighted in the calculation of the FSIQ (what most people think of as the IQ). I would not worry about a rapid-fire test process precipitating melt-downs. The majority of the test is quite gentle, and even fun for many GT kiddos, though there may be moments when she feels challenged because discontinue rules require testing until a specified number of failures have occurred. If the examiner is aware of her astigmatism, she will be able to take that into account in her interpretation of the visual parts of the test (the VSI, FRI, and PSI subtests, mainly), most of which should not be notably affected by her vision. (I assume she has and wears corrective lenses?)
There are no legitimate sample questions for individually-administered IQ tests and achievement tests. The best way to prep her is to make sure she is adequately rested, fed, and comfortably clothed for test day, and to clearly convey that the outcomes of the test in no way reflect on her value as a human being.
Testing is for the purpose of understanding oneself better. As long as she does not experience this as a high-stakes gateway to something, I would say that this is all the prep you need or should have. It sounds like you have already dealt with the high-stakes aspect with admission to your county program.
At age 7, the test ceiling on the WISC-V should be plenty, for the majority of children, even many 99th %ile children. The test is normed to age 16-11, so there are items designed to push a high school-age student. As a recent release, with the most current norms, it should result in more accurate scores than older tests.
In terms of achievement testing, the WIAT-III and KTEA-3 both have linking studies attaching them to the WISC-V, which makes statistical comparisons a bit easier and more reliable. On the other hand, both have ceiling issues for selected subtests.
On the WIAT, the reading comprehension subtest notoriously underestimates GT children, because of the grade-level item sets, which are pretty fixed (you can drop back for lower-functioning students, but not advance for higher-performing students). Oral reading fluency also has grade-locked item sets, but is usually not as much of an issue. At second grade, she would not be administered the essay composition (extended, multi-sentence writing) subtest at all.
On the KTEA, reading comprehension has item sets with decision rules, which does allow for continuing to higher levels, beyond the grade-level set, which would be a plus for high-functioning students. The written expression subtest, on the other hand, is administered in strict item sets. Then again, at least the second grade set includes the possibility of essay writing, so, unless your child is an exceptional paragraph-writer, ceiling effects may not come into play.
From a ceiling standpoint, the WJIII and WJIV (latest version, just out this past fall) have the most generous ceilings, as they do not have the item set restrictions. They lack the linkage data, being from a rival publisher, and also of very recent publication date. I find them to be slightly less developmentally engaging, though that will be highly individual to your child.
Given your description of your child, the most important factor is probably going to be the clinical skill of the examiner, especially her ability to build rapport with your daughter, and set her at ease.
