As for the speed, my home internet connection is very fast now but the blockchain downloading (syncing) was VERY slow. My download bandwidth was being used at 2-3% only.
Core downloads blocks from many sources and it is comparable to a torrent. The downloading itself is fast but the validation process is very slow. Its not a problem with the internet speed but rather, your hardware speed.
But if there is a security risk, then maybe is there a Bitcoin Core command or something to validate all its database against the actual real blockchain?
There really isn't a security risk. The security risk it has would be the same as a user downloading any other files from the internet. If you want to be extra safe, use -reindex when you start Bitcoin Core up. Be warned that it could take days for it to be completed.
the chain that will be chosen as the dominatn one is that with the most nodes running up. So if you support Bitcoin Core you must run a Bitcoin Core node and accept and check transactions with it.
good luck
No. Nothing is chosen in the hardfork. Both chain will still exist.
If you want to help other to download the whole blockchain just make sure your node accepts incoming connections.
No. Outgoing connections also helps others to synchronize their blockchain.