Yes.
Windows 98 second edition
You do not need to download DirectX 6.1. All versions of Windows from Windows 98 SE on up include DirecTX 6.1 or newer. If you have Windows 95, you should download DirectX 8.0a, available at the link below.
No - after Windows 98 came Windows Me & Windows 2000 - then Windows 7.
1999.
Second Edition. There were two separate releases of Windows 98. The second edition is slightly updated and has improved USB support.
Yes.
yes
USB is supported by Windows 95B and 95C, Windows 98, Windows 98 SE, Windows 2000, Windows ME, Windows XP, Windows Vista, and windows 7
"Windows 9x" is the collective term for all operating systems in the 16/32-bit hybrid family. These are Windows 95, Windows 98, Windows 98 SE, and Windows ME. Windows 98 refers to a specific member or two members of this family.
Since there isn't a UNIX 95 or UNIX 98 per se, I think you are referring to Windows 95 or Windows 98..
The last version of Adobe Flash Player that will run on Windows 98 is 9.0.246.0.