Minecraft – How to add a scoreboard listing the health of other players when I press TAB

You can't specify a radius for the command /say. You should learn json and use /tellraw instead. If you want to send the message to everyone within a radius of 100 blocks, do /tellraw @a[r=100].

Basic JSON formatting: /tellraw @a[r=100] {text:"Hello world!"}

With colours: /tellraw @a[r=100] {text:"Hello world!",color:"aqua"}

These colours have different names than wool colours, for example cyan is dark_aqua. These names can be found on the Minecraft wiki here.

You can do lots more with JSON than you can do with /say. Check out this awesome json generator: https://www.minecraftjson.com/

The commands /gamerule commandBlockOutput false and /gamerule logAdminCommands false should work. The first command blocks the output from chat, and the second blocks it from logging.