I don't think it's economically viable to manufacture scalably.
What is this person going to do when he eventually puts it on Kickstarter or something and thousands of people order them?
Much better, I think, to use industry-standard assembly practices that are scalable using standard pick-and-place technology.
I would change the PCB to something thin like 0.8mm, change the TQFP AVR to a QFN package, change the passives to 0603 or 0402, mount all the components on the board in a standard way compatible with established manufacturing practices. And just accept that may add to overall thickness.
Maybe keep a cutout for something like the battery, because the assembler may be hand-soldering them anyway and they may not be pick-and-placeable, so you might be saving thickness and not gaining any manufacturability by keeping the cutout.
If you launch a Kickstarter campaign for an electronic hardware project and you're hoping for big results, backers will be looking for DFM, an understanding of manufacturability, and financially viable high scalability without the need for a redesign or a big blowout to the timetable or budget. (Or at least that's what they *should* be looking for, if Kickstarter backers had a decent sense of skepticism.)
I also don't like the questionable "let's get rid of all the decoupling capacitors and LED current limiting resistors" engineering approach. It doesn't add engineering credibility to the project IMO.
And a LED with no current limiting resistor is probably draining the finite charge capacity of the lithium coin cell very fast relative to what a LED current on the order of 1mA (through a 1k resistor) will.
Also, put a standard AVR ISP header on it (even if you just pogo-pin it and don't solder header in) because you'll want a rapid-programming jig to put the firmware on it for high-scale manufacturing. You can think about these high-volume manufacturing issues even right from a small start - it will save headaches later. (And a serial header with an FTDI-compatible pinout or whatever, even if you don't solder pins in, people will be able to program it and hack it later and develop their own code with the Arduino IDE.)