Hey Angus,
This is a super interesting question. My company has a very short-cycle transactional sale as well so I can relate to many of the questions you are asking.
High-level, I personally think engagement programs are not the best fit for the activities you describe. You could make it work technically, but it is a bit square peg/round hole.
My onboarding program (which is also 7 days BTW) is a similar pattern of emails and calls with different email streams that are behaviourally triggered. I have it as part of a single program in Marketo.
I think your idea of synching each of the outbound activities to a child campaign in SFDC is a great one (wish I'd thought of that). You could synch the parent program in Marketo to the parent program in SFDC, then have child campaigns for each activity and use a controller within the Marketo program to assign leads to the relevant child campaign in SFDC as they progress through the steps. You would need to include Marketo smart campaigns to update member statuses in the SFDC child campaigns based on the outcomes to ensure they are marked as responded etc.
For triggering phone calls or other tasks, you can use the "Create Task in SFDC" flow step. I make heavy use of that one and it is the way you can coordinate your automated and human-executed activities into a tight multi-stream nurture.
E.g.,
In terms of the longer-term drip, whether you have a single or multiple engagement programs would likely depend upon a number of factor, including how different the content pieces are (for instance, could you just use segmentations and dynamic content within a single email? or are they entirely divergent streams?).
For tracking and reporting the segments separately it's probably a lot easier to have them in separate programs so you can see the metrics for each. But if the changes for each industry are minor then the overhead on maintaining separate programs could be greater. Hopefully others will chime in who have tackled this problem with what they have done, as we don't use engagement programs for that either.