I have not downloaded k2 yet. I want to be sure that I understood correctly. I am working on a website using joomla 2.5. I am one of those people that has to do everything in css and am relatively new to component based projects like Joomla. I read that "K2 was built as a complete replacement of the default article system in Joomla!" And that sent a wave of relief .I also read "K2 has "sub-templates" (aka micro themes) that you can edit via php and/or css to create the layout of the content. Then in the category screen you can choose one of the sub-templates for all the articles in that category."

Question:
Will writing sub-templates" (aka micro themes) be as simple as adding custom.css code found in joomla?
I have bought a third party template from joomlage and there is no mention of k2 support. Will I have to give up this template or are there any conceivable potential conflicts that can arise?
User have creative ways of inadvertently destroying a well thought out design when using their admin editors. For example when they cut paste from word docs. Does K2 in any way help with this issue? Is there some good practice which can be recommended.

Yes, your H tags and links could differ from styling in template CSS. I need to change it on every new Joomla instalaltion, when i use K2. (i mean to adapt K2 categories to whole website)

Rest is OK, K2 doesnt impact other classes on website.

Second question is not about K2 or Joomla, depends what HTML editor you install and use. See on JED what are most and best reviewed.