I agree that Sombra could work as a support, most of her kit is supportive in nature anyways, so with a few changes to invisibility, hack and EMP I could see her working in the support roll. Some of the changes I was thinking about would be.
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Hack reverted to OW1 levels against enemies (with maybe a shorter duration on tanks), but adding a heal over time similar to your suggestion, and lengthening the cool down.
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Add a heal over time component to EMP, while decreasing the frequency of EMP.
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Allow Sombra to remain clocked while hacking health packs, but revert it back to de-clocking you if you try to interact with another player (either by shooting them or using hack)
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Decrease her SMG’s damage (since she would be a support and doesn’t need the frag potential anymore)
I think that those changes would work while keeping her identity.
As for Symetra. I still believe that she should remain a damage character. All she needs to be less annoying is a small change to her turrets (like giving them a 10-20 meter deploy range instead of a fire it across the map range that it currently has), and then they could focus on her damage numbers to keep her from becoming to OP and bring a better balance to the character.