Thanks for raising this @rtoma - we certainly want you to stay happy & continue hacking on plugins 
First, there is this issue which also relates to this.
Let me provide some thoughts on your points:
1- we'll be following up with some recommendations on this and see if we need to change/add anything to make this easier for you.
2- gemifying your plugin is really not that difficult, it's basically just adding a .gemspec file.
3- not sure I understand what you mean here. each logstash installation is self-contained. plugins installed in one instance are not visible to another instance. normally only one version of a plugin is installed. you can install a specific version and you can update to latest version but there is no concept of having multiple versions installed and choosing a specific one - you simply install a specific version (or latest) or you update to latest.
Colin