On RadChart export - it has never been intended to work with vector graphics and produces only raster images, so you may need to go with the RadHtmlChart.
You can find example of exporting it here: http://www.telerik.com/support/code-library/exporting-radhtmlchart-to-png-and-pdf. It uses a third party tool (in this case Inkscape) to generate a PDF or PNG from the SVG string. Depending on your needs, you may need to tweak it.
On avoiding user interaction - you would, indeed, need to run the browser on the server in order to get the rendered HtmlChart. You can consider using other tools to run the browser and get the needed data, like PhantomJS: http://phantomjs.org/.
Whether this is a reliable solution - I do not see reasons why it would break, especially if the code that runs the browser instance cleans up after itself (i.e., closes the browser). All export operations put load on the server, yet running a browser with one or a couple of charts should not slow it down significantly, generating a large PDF is likely to require more memory and resources. If this is still not an option, you could consider requiring user interaction so that the charts render on the client's browsers and they only send you back the SVG string.
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