Read up the docs on devportal and try out different API calls. I might write another startup guide targeted for newer devs. I think this was more for the devs familiar with the Hive blockchain overall and how it works.
The code reads off of the chain so yes.
The block number won't change fast. There is a minimum of 3s between each block produced and we pull the last block less than that. But, another extra check to make sure the increase is not more than 1 would be better.
A testnet would be ideal but using mainnet for small things wouldn't matter. Mainly because it doesn't involve financial transactions.