Is it possible to decrease focus distance to 3-4 centimeters on fixed-focus OV2640 camera modules which sell with Chinese ESP-32 CAM or M5Stack ESP32 CAM?
They are set up to infinite focus out of the box.
Electrical – Shortest focus distance of OV2640 camera modules with fixed focus
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The kind of CMOS camera modules you are asking about all have global shutters (electronic shutters, not focal plane shutters). You control them through their bus, typically some sort of serial bus like e.g. I2C.
You can find modules where you have control over both frame rate and shutter speed.
I very much doubt any camera module has a separate shutter pin. You need to implement the bus protocol anyway, or there would be no way for you to get the image data...
I might (doubtful) want to try and sell this device, so I can't simply take some Logitech web cam and use it due to obvious licensing issues. So that's a huge constraint.
Basically you will be creating an IP Camera. The Raspberry Pi is probably the cheapest and easiest way to prototype this, however there are other boards like the Beagle Boards.
Once you get a prototype going, then you may consider creating your own all in one device that uses an ARM or DSP processor. For example I would probably use some type of Serial JPEG Camera Module, a cheap microphone, and the cheapest ARM processor that will fit these requirements. But for a prototype, the Raspberry Pi and a cheap usb web cam is probably the cheapest and quickest way to get started and get your software going. You may even to be able to find a cheap no-name usb camera from china that will work for this so you can resell it in small quantities.
So as far as getting the frames to the MCU, there's two main issues here: (a) the choice of camera and microphone, and (b) the drivers for connecting the cam/mic to the Pi's USB port.
Here is the huge list of devices that work with the Raspberry Pi: RPi Verified Peripherals. The USB Webcams section lists both working and problem units. Along with a bit of extra info. After you find a unit that fits your requirements (price, etc.) I would try to double check via Google to verify someone has used it and it does indeed work; although this is probably unnecessary.
I'm sure there are other units that work that haven't been tested, the two things that will help you is to make sure it is Linux compatible, and that there are ARM drivers available.
There's also the issue of A/V encoding as well as synchronizing the video and the audio feeds together.
As I mentioned in my comment, the RP really won't have any issues handling this part. It has more than enough processing power to handle most all A/V formats.
the Pi (which would be running GNU/Debian linux) will have the right drivers to ingest the streaming frames and send them off to a tool that would then be able to forward them on to a WiFi or Ethernet adapter
Basically the RP is just going to be a linux computer that is connected to the internet and has the camera and microphone plugged into it. You will install and configure libasound2-dev (for the audio) and FFMPEG to stream everything.
Then this will just be like any other server online (you may need to configure your router and port forwarding etc. to get it to be visible on the web,) and according to this, you and other computers will just access it by going to h**p://YOUR_WEBCAM_SERVER/webcam.mjpeg
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The existing lens is adjustable, you just need to turn the knurled ring around the lens, as shown here:
See whether that lets you focus close enough. If not, you might be able to find a different lens in the same kind of holder.