Searchable comments have been requested many times over the years; at this point I doubt it will ever happen.
You can increase the number of posts per page in your settings. Hover over your name at the top of the page by the search box, click "User Control Panel" in the menu, click "Edit Options" on the left on the page that comes up, and scroll down to the "Number of Posts to Show Per Page" option. This applies to both forum posts and addon comments.
However, the first page of ArkInventory comments still has stuff from 2012 on it, and since the author appears to be active, I would sincerely hope that any bugs reported earlier than 2012 have all been long since fixed. If you're having a problem that isn't mentioned on that first page of comments, or in the FAQ and wiki pages linked in the addon description, I'd say it's safe to go ahead and just post a comment.