Can I use plastic bag as a personal item
Yes (unless the airline specifically forbids it). Plastic bags are pretty normal on flights: for example a lot of duty free stuff gets carried that way.
Weighing practices vary from airline to airline. I do fly a lot but my carry on was only put on an actual scale a few times, and then mostly by LH group (Lufthansa, Austrian, Swiss). A normal looking backpack is unlikely to draw attention.
If you actually need to weigh it AND they deem it to heavy, it's ok to pull out an item and carry it as "personal item", wear it, or just shove in your pocket.
Some of that dance is just silly: One of the few times I got weighed was in Hong Kong (hot!) flying to Boston (cold!). I had my jacket in my bag and the check in agent gave me a stern lecture about being about 1kg over. So I offered to pull out my coat and just put it on. She shut up and let it go.
Here is the ultimate workaround :-) https://www.youtube.com/watch?v=hviF2sr4aq0