How do we understand if caliper brakes can be installed on the frame?
Caliper brakes attach differently to V brakes, they have one pivot bolt in the centre rather than one each side. In your first photo there's a bolt that is in the right place although it's there for a mudguard/fender not a brake:



A caliper brake actually needs a hole right through there, and has a long bolt that goes right through. On your bike there are two chainstays there, with a brake bridge across. The problem is that it might not be strong enough to take an effective brake, since it's probably designed just to hold a mudguard. It will look something like this:



At the front, your fork is probably drilled to take a caliper brake and that part will definitely be strong enough. If it's not drilled you can probably get away with drilling it.

One alternative is to use cable pull adapters, the most common model being "travel agent".

It's likely to be cheaper to sell that bike and buy another one that has drop bars, than to convert the bike you have. Even if you use second hand parts for most of it, the new extra parts you need will add up.