Little update: I just found a solution which allows full post display for categories WITHOUT BREAKING THE EXCERPT FUNCTION. This is better than the solution Prasant suggested above.
/views/global/_content.php outputs the posts for us and decides to call either
Line 16 determines which pages get the full content. It reads:
if ( is_singular() || $is_full_post_content_blog ) :
Adding another OR statement can easily add categories to the list of pages which get the full posts. This does NOT break the excerpt function, so the archives and other things can still use excerpts.
if ( is_singular() || $is_full_post_content_blog || is_category() ) :
I tried to also exclude archives by year, month, and day, but this FAILED and I couldn't figure out why.
if ( is_singular() || $is_full_post_content_blog || is_category() || is_year() || is_month() || is_day() ) :
If you don't want any excerpts anywhere on your site ever, make the mod Prasant suggested to
views/global/_content-the-excerpt.php. It's a bit sloppy and confusing to mod
the_excerpt() to output the full content, but it gets the job done.
If you only want to give categories the full posts (the original request from Artemy) then make my mod to
/views/global/_content.php. It's a more honest solution which still uses
the_excerpt() for their intended purposes. The toggle option in Customize > Blog > Full Post Content on Index is not broken and the archive excerpt displays are unaffected.
Hopefully somebody else can figure out the trick for archive pages.