Pass string to Jenkins job from Slack
The plugin you found doesn't have a 1:1 ratio between Heroku instance with build jobs since it takes the job name as parameter, so you should be able to run any number of jobs from a single Heroku instance.

As for passing a space in a job parameter, the word split is made on spaces so quoting won't change anything. You could possibly try to use the HTML spacing character %20 and see if it works?

Personally I would try to use the plugin to run the correct job with the associated parameters directly instead of going through filtering jobs.