Negate the need for blockchain explores and full blockchain syncing for simple queries Rather than download the full blockchain or a pruned version; would it be viable to peer-to-peer ask 'what is this block' or 'what is this tx'?

We download the full blockchain from several peers so why not ask peers for just on piece of info?

Is there a javascript/node.js library that does this?
That's exactly what your bitcoind client does. It asks for specific blocks in network calls and if you wanted to, you can create a simple client that asks only for certain tx's and blocks like you said.

That is in fact what most SPV wallets (phone-based wallets and some others) are doing. They ask for block headers only and then ask for transactions related to those blocks to prove that certain transactions exist.