Syncing openGL rendering with c++ game loop on android

I am building a game like application using android NDK and openGL ES 2.0

So far I understand the concept of vertices and shaders and programs.

The main game loop would be a loop in a single thread as follows

step 1. Read all the user input

step 2. Update game objects (if needed) based on the input

step 3. make draw calls for all the objects

step 4. call glSwapBuffers

then loop back to step 1

But I ran into various confusions regarding sync and threading so I'm listing all the question together.

1.Since open GL draw calls are asynchronous the draw calls and glSwapBuffers may get called many times before the gpu has even rendered actually a single frame from calls from last iteration of loop. Will this be problematic? buffer overflow or tearing ?

2.Assuming VSYNC is enabled then does point 1 still causes problem?

3.Since all calls are async how do I measure the time spent rendering each frame? glSwapBuffers would return immediately so how can I know when was the frame actually done?

4.loading textures will occupy space in the ram is checking free memory before loading texture standard way or I should keep loading textures until I reach OUT_OF_MEMORY_ERROR?

5.If I switch to multithreaded approach calling just glswapbuffers at a fixed 60 times per second without any regard to the thread which is processing input and giving out draw calls then what is supposed to happen?

Also how do I control the fps in game loop? I know the exact fps depends on a large no of factors but how can you go close to that