How to change WeeChat’s default nick

This depends on the IRC server. Normally, the WHOIS query would be processed by the server you are on. But on IRC, servers only know idle time and away status for directly connected users. (This is to avoid a lot of unnecessary traffic between servers.)

To see idle time, put the nickname twice, like this:

/whois foo foo

This is aliased to /WII by default:

/wii foo

It is also in the builtin help:

/help whois

This will cause the query to be forwarded to the target user's server, which knows the idle time.

Note that on some networks, cross-server WHOIS queries can be heavily throttled (example: freenode).

You can use irssi's -n option to specify your nick

irssi -n test123

or you can go to the bottom of your $HOME/.irssi/config and in the settings section, change your nick