No, we don't have the technology. Once cancer cells are systemic (metastatic), they can end up in bone or brain, where it can be extremely difficult to detect or treat. Cancer cells only need one rogue cell, and takes about 2 years to grow to a size that appears in tests.
Sadly, cancer can also mutate to avoid the PD-L1 pathway.
About 100% of older people have cancer. It's just that many of those cancers are tiny and slow growing, such that they don't cause any serious clinical symptoms and often aren't even detectible with routine screening tests.
I've never heard of that, where did you read about it?
You’d have to come up with a definition of cancer free. Cells replicating poorly resulting in a cancerous single cell happens all the time and gets cleaned up.
Possible? Sure! Individual evaluation of every cell in your body is possible.