1) Largest drive must always be parity drive in any RAID system. I myself just copy my data over and remove the old drive and recheck, or you could let parity rebuild the files.
2) 16GB should be plenty for what you listed you only need more RAM when doing VM's and maybe some complex docker setups.
3) Good question not 100% sure
4) There's plugins for these things or use a docker image
5) Depends how you configure it, for the reason you list alone I configure all my drives as single store so data is not spread across (easier for me to offline backup/update and recovery if multiple drive failures)
FYI: SECURITY IS NON-EXISTENT so i'd suggest VPN to your (unraid)owncloud if thats what your looking to run, hopefully you have a newer router which has OPENVPN support to make things simple