Does this sound like a good plan?
The layout looks good to me and well thought out, are you saving the blade edge for something different? rather than the NExT between metacarpals, just curious, not an issue.
Maybe I should wait with xG3 installation and get some sensing magnets (from where?) instead.
Re. sensing magnets, sorry, I can’t help you there.
How close to each other can I put the xSeries implants without interference?
That’s tricky, it will largely depend on the read range of the reader you are using and your physical approach to it, For safety, 5mm is the MINIMUM distance to prevent / reduce chance of breakage, but for crosstalk / interference you would likely want to get further away with same frequencies ( xM1 and Spark for example), also as you have the xLED and xM1 in close proximity, as you have likely planned, this is where you want both read at the same time. Because Blinky 
Also important thing, I like to shoot a bow. Is it safe to install implants in the webbing between the thumb and index finger if I shoot a bow?
Archery, 
not a problem, It sits comfortably under the shelf of the arrow rest, I never even thought about it until you asked the question, I just went and let loose a couple of arrows, and couldn’t even feel it! 
