I have looked over your question in detail, I have reviewed what is published about Err 70 in EOS cameras of the same model line as your canon model. It seems to me that you have done, largely, what you should try.
How old is the battery? older batteries may cause problems, to summarize.
Have you tried removing the battery, while the camera is On, for a prolonged period, such as 1 hour or 2 ? I recommend that, if it has not been done already.
The Err 70 is officially an 'image related' malfunction signal. That might mean, unfortunately, that encoding circuitry in the camera is self-detected as not working properly. (This might eventually result in scrambled images, or no visible result from taking pictures.)
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