Why is an invalid pubkey accepted in the blockchain?
Why is an invalid pubkey accepted in the blockchain?

It was intended as a message, not as a public key.
It is accepted in the blockchain because it is a valid output script, whether or not it is spendable is not relevant.

Where does the address 1Du8MpTTV4VrQMHtgP4mG1Fq9q3ykb3L8h come from?

That address is a result of the software blockchain.info uses to generate an address from an output script. Why or how that works is something you'll have to ask whoever wrote it.