1. How do I remove Jumi completely?
2. Also, is there a way to install the plug-in, the component, the module separately. Let's say I only need the plug-in...??
First: I do not know why your installation crashed on 1.5.8. We have verified it many times without the problems. But sometimes queer things happen...
I do not write here how to uninstall Jumi correctly. But only crashed uninstallation.
Suppose you have manually removed from your server all directories with Jumi plugin, module, component.
You have to manually alter your Joomla! database too:
a) the whole table jos_jumi
b) record jumi in three tables: jos_components, jos_plugins, jos_modules
The way is. We, developers, do not provide it and support it. Because not all features of module and plugin will be available without the Jumi component.
But if you need it despite this warning do this:
a) extract from distribution package directory admin/plugin
b) pack it into zip
c) install it
The same applies for the module: admin/module
Separate component installation can be done too, but it requires much more steps. Better to install the whole package and them uninstall plugin and module.