Rebecca,
These are good questions!

Since we do not have the Modern Times guide written as of yet, I can't fully answer your question on high school credit for that guide. However, at this point I would think the guide would be fairly easy to make credit worthy in most areas. We will definitely be keeping this in mind as we write it.

It is our intention for the history portion to be credit worthy. The Bible portion would also be easily able to be credit worthy, with a bit of additional Bible reading added each day, as we intend at this juncture to schedule AIG's Pilgrim's Progress within the guide and also are looking to schedule Beyond Beautiful Girlhood for girls and Practical Happiness for boys.

The literature portion could be credit worthy through Drawn into the Heart of Reading for 9th grade if desired (with high school level lit.), along with Rod and Staff grammar, dictation, and whatever writing program we end up scheduling.
The science portion may need some beefing up, however we are going to be covering an introduction to physics/chemistry (which often is listed as Integrated Physics/Chemistry on a high school transcript). Until we have more fully nailed down our resources for science, I won't know for sure. Whether the science is "enough" will also be dependent on your child's long-term plans, as to how much science he will need in preparation for any future degree.
The math will include an Algebra I option, so depending on where your child is at for math it will be credit worthy, but it is possible that your child may need Algebra II if you decided to do Algebra I early.

Anyway, that may help give you a fuller picture of what is ahead and how easily you'll be able to use it for high school credit.
One thing that I will share is that our plan for our own son who is in 9th grade in high school right now is for him to do geography in grade 9, a one-year overview of world history in grade 10, the first half of American history with government in grade 11 (similar in time frame to what is going to be within Rev. to Rev.), and the last portion of American history with economics from approximately 1900 forward in grade 12 (simlar in time frame to what is going to be within the guide to come after Rev. to Rev.).

Should we end up writing guides for high school, this is likely the scope and sequence we would follow, allowing families to choose whether to use our younger guides and beef them up to get their credits, or whether to start the geography guide in high school and follow that plan forward instead with no beefing up. Either way would work, depending on the family goals, because in the end you'd want one credit of American history, one credit of world history, one credit of geography, 1/2 credit of government, 1/2 credit of economics.
The geography guide will be written with the goal of it being credit worthy in the areas I listed above.

So, no matter whether a family is using it prior to high school or for high school it will be worthy of credit if needed.
Blessings,
Carrie