Verner's law is more complicated than Grimm's law. You should read the Wikipedia article in detail:
http://en.wikipedia.org/wiki/Verner%27s_lawIn short, it is a way to account for certain exceptions to Grimm's law. So if Grimm's law works, then that's probably enough for the question. If you find exceptions, then Verner's law should account for those. There may also be a subset of cases where Verner's law does something that seems unrelated to Grimm's law.
In short, Verner's law accounts for exceptions where you find a voiced form where you would expect an unvoiced form. The pattern has to do with stress:
the apparently unexpected voicing of voiceless stops occurred if they were non-word-initial and if the vowel preceding them carried no stress in PIE
To be clear, there are no exceptions to Grimm's law because of this: Verner's law is an unrelated change that merely masks the regularity of Grimm's law-- it still happened regularly but then in some cases there were other changes according to Verner's law that also had an effect.
(The Wikipedia page notes how it is unclear which happened first, and that it doesn't really matter.)
Finally, there's an unrelated change of z>r in certain morphological paradigms that results in pairs like "lo
se"/"forlo
rn" (rare in modern English but found throughout most of Germanic), so the /r/ can appear to be due to Verner's law (because that's why it was /z/ rather than /s/, and only /z/ became /r/) but really Verner's law itself didn't generate the /r/ form. Often, however, these /r/ instances are focused on as an interesting effect of Verner's law.
For more info, see the Wikipedia page. It's a lot of details, but it makes sense after you work through all of them.
As for your answers, they look fine to me. Note that for Verner's law to explain the last one, you'd need to check the stress pattern, which (at least from what you typed) isn't given in your data.
There are also some complicated details regarding the consonant clusters (why octo-acht with /t/ in each?) but I would guess that's beyond what you are expected to know/analyze at this point. (As a simple observation, we can just note that consonant clusters appear to involve some exceptions, which is not surprising.)