Multiple stack abilities and casting spells in that time
Yes, it is possible to respond to one trigger after others have finished resolving.

After Splinterfright's ability resolves, the active player (you) gets priority to play spells/abilities. See rule 116.3b:


  The active player receives priority after a spell or ability (other than a mana ability) resolves.


At this point, you have an opportunity to cast Dark Ritual. If no player has any further responses, then your Ritual will resolve, followed by Endless Ranks of the Dead's ability.