That sounds like a faulty Uno board; with an 328P processor (as used on an Uno), the L-LED can not get brighter and dimmer, it can only be on or off.
If you insert one of your 328P chips with bootloader in the breadboard and connect a LED plus resistor between pin 19 of the processor and GND, do you get the flashing of the LED as described in post #4? Leave all wiring to the Uno board out but you need to supply 5V in some way.
Because your Uno might be faulty, try to use a processor that hasn't been in Uno yet.
If your Uno does not use the CH340 (used on many cheap clones) TTL-to-USB converter (the chip closest to the USB port, you can try the Loop-Back Test Instructions. The official Uno uses the 16U2 which is a square chip with legs (pins) at all sides).
What did you buy; you can take a sharp photo and insert it in a post if you don't know.
Don't worry about my time 