Not at all. If they are contracted to deliver an email template, the fact that it renders correctly shouldn't be an extra charge, that is the whole reason they are being hired. Sounds like a sloppy agency.
I've made a graph generator for email. Figure now is a good as time as any to let it out in the wild.
Check it out here: https://thesimplepostmaster.com/
Multiple types of graphs and responsive data tables, in easy WYSIWYG format. Base code is here if you want to play around with it:
Let me know if you find any bugs!
For the most part, no. All dynamic content is rendered server side so by the time it gets to the client it will be static. The only thing I can really think of is something like moveable ink, which would be obvious that elements are dynamic.
Could do something like this:
Use mso to hide and show different things.