Combining 2 MySQL queries with IN clause does not work
You have basically two problems here,

Your sub-query is returning row-based results and you need comma-separated results.
You could fix this issue by using the GROUP_CONCAT(value SEPARATOR ',') like this:
...r.reactable_id IN
(
select  GROUP_CONCAT(id SEPARATOR ',')
from    (select * from comments...


But you won't see any result due to the next problem...

Different types (reactable_id is an INT value and the GROUP_CONCAT result is a STRING value. So your final solution is to rewrite your query using JOINS instead of sub-queries.
You can see this post to guide your steps:
https://stackoverflow.com/questions/6135376/mysql-select-where-field-in-subquery-extremely-slow-why