Hi Bombers, welcome to the forum.
Thanks for sharing your experience installing the replacement head unit, good to hear it's mostly going well. I've been tempted but shied away from that kind of upgrade due to the level of integration with the car's other systems (Convers+, parking sensors, etc) - I don't want to lose anything I currently enjoy with the factory Sony unit, even if I would get these other cool features...
Can't help with the adaptor for parking sensors, sorry.
Regarding the microphone, I think these replacement units normally have one built-in (does it have a hole labelled MIC?) so it may not be possible to take an external input from the stock one.
For offline maps, here are instructions from Google, do these steps work?
Download areas and navigate offline
I also have a couple of dents and dings in my car and have been looking at ways to fix them. Have a search for paintless dent removal or PDR on YouTube, there are good videos of the techniques to push and pull dents out of body panels without needing to repaint. It's not easy but looks achievable. You can buy PDR glue pull kits off ebay to do it yourself inexpensively. I've been intending to give it a go but have been holding off until my schedule clears a little, otherwise it's likely I'd order it and then it would sit in the garage for 18 months ;)