Replace or append in configuration file with sed - sed

Question

I would like to replace or append in a configuration file like sshd_config:
Key1 value
#Key2 value
The idea of the command is:
$ cmd Key1 home file
$ cmd Key2 house file
$ cmd Key3 flat file
So the resulting file is:
Key1 home
Key2 house
Key3 flat
Any help is more than welcome.
I have taken this as an example but the one that comments and uncomments is not properly working.
Besides I have managed with other options but only for comments or uncommented lines and I want everything in one command if possible.
sed '/^Key\s/{h;s/\(\s\).*/\1newvalue/};${x;/^$/{s//Key newvalue/;H};x}' file
This one gets if the Key exists but, how do I append if it doesn't=
sed -i 's/^#\(Key\s\).*/\1newvalue/g' file
Thanks a lot. I have tried to understand sed but it is quite complex the different spaces and I don't know how to get with # or without.
Edit: Stdout output with -i inplace
$ sudo tee -a /usr/local/bin/conf-space-replace-or-append > /dev/null << 'EOL'
#!/bin/bash
awk -i inplace -v key="$1" -v val="$2" '
($1 == key) || ($1 == "#"key) { $0 = key OFS val; done=1 }
{ print }
END { if (!done) print key, val }
' "$3" > /dev/null
EOL
$ sudo chmod +x /usr/local/bin/conf-space-replace-or-append
$ sudo conf-space-replace-or-append Port 22 /etc/ssh/sshd_config

Answer 1

sed is for doing s/old/new on an individual line, that is all. For anything else you should be using awk for clarity, simplicity, portability, efficiency, etc., etc.
Just put the following in a file named cmd and execute it as you show in your question.
awk -v key="$1" -v val="$2" '
($1 == key) || ($1 == "#"key) { next }
{ print }
END { print key, val }
' "$3"
The above will delete the existing key+val if present and always appends the new pair to the end of the file. If you'd rather keep an existing key in it's original position in the file and only add new key+val pairs to the end then that's just a tweak:
awk -v key="$1" -v val="$2" '
($1 == key) || ($1 == "#"key) { $0 = key OFS val; done=1 }
{ print }
END { if (!done) print key, val }
' "$3"