Sure you can do that. You can use as many trains on a travel day as you want.
Titlis is a mountain, so it depends a bit how close you want to get. You can surely use the train for part of the way, e.g. until Engelberg, which is included. And it looks like some/all of the lifts from Engelberg have a 25% discount, if I understand the validity map correctly.
Hi
To answer your first question…...You need not to utilize more than one travel day as you are traveling on the same day. However, you just need to add more number of trips on the same travel day and you are ready to go.
Regarding your 2nd question……..If you wanna go to Mt Titlis, You need to go to Engelberg first and from there you need to purchase your tickets to go to the Titlis which are not covered under your Eurail Global pass. However, from Zurich you can travel to Engelberg by utilizing your Eurail Global Pass. There won’t be any problem with that.
I hope your purpose is addressed.
Thanks and Happy Traveling.
with profound regards
Razib from Bangladesh