I would present a strong business case to change email service
Your client says that they can't move away from the current email provider. So give them a good reason to switch! My experience is that, when presented with clear benefits argued delicately, no company is wedded forever to a single service or piece of software.
As such, I suggest that you build a case for moving their email service to MailChimp, which has rich templating features designed to do exactly what you describe:
You create the original template with editable repeating regions. Users with no knowledge of HTML can then add and edit only those regions (and create multiple ones in the case of your seven headers scenario), then send campaigns without worrying about breaking the master template, which is editable only to admin users.
In my opinion, this would be a faster, cheaper, and less stressful solution than attempting to hack together your own templating service that ties in with Silverpop. It may take you 50 hours to build a good templating engine that works around the weaknesses in their existing software, or to hack together a CMS to kind of do what you require. The result would likely be functional, but not as powerful or polished as more mature software like MailChimp.
When a client constructs a set of limitations, it's worth questioning why they exist in the first place, then attempt to break them down or remove them entirely. Often, I've found that the limitations prove to be artificial. When you actively poke and question why they exist, you often find companies stuck using software that's not fit for purpose because 'it's always been that way' or 'that's what Big Dave set up before he moved to Thailand to open a bar'. (Honestly; someone really told me that!)
Show them that MailChimp (or whatever service you're comfortable recommending to them) is cheaper/more productive/better for templating/easier to use/better documented/trusted by The Economist etc. and see what they say. Show them that that's what their competitors are using because it offers x, y, and z. Remind them that a modest change can bring big benefits.
It's our job as Web consultants to put forward the best solutions we can -- the things we would be happy recommending to our friends and comfortable using in our own businesses. Sometimes that simply means telling clients that their preconceptions about their entrenched software being best are wrong.