Is it not strange that all models have similar problems, I try to find same problems on other brands and it's not as easy as on Lumia, is it sh#* assembly or parts or is it all software
In the case of issues with battery to pin contact with removeable batteries no. I've had the same issues on other reburbished products from other brands with removeable batteries, and I use an e-cig and the battery contacts need constant cleaning or the product ceases to work. The combination of build up on the connectors, and loosening of the springs on the pins, tends to produce issues over time where the contact is not as good.
I had a phone at one point I had to drop, on its side, from a specific height, to get it to boot because of this, lol.
This is more or less just a problem with removeable batteries versus device age, AFAIK. It is very worth using alcohol and giving your lithium battery products a good clean around the battery contacts whether its an e-cig or a phone, and using something slightly abrasive to clean the ends of the pins.
And as far as lose pin springs go, well the best you can really do without replacing them, is to try and put something in there to push the battery up firmer into the springs. A new battery might help too, if they contacts on the existing battery are uncleanable.
As for firmware issues, also not particularly strange. Unfortunate, but not strange. Firmware issues with various products on shipping are not uncommon, and are usually patched over time.