For myself.. most of stay will be at a resort in Terengganu.. Will that influence choice of TelCo?

You should be fine with any operator then. If you have Bukit Tabur on your list in KL, the trail is officially closed (if I am not mistaken). There were too many serious injuries and a handful of deaths on the trail due to inexperienced hikers (some parts involve climbing) trekking to the peak. But there is nothing preventing you from hiking it other than a yellow tape blocking the entrance.

Maybe try going to Cameron Highlands too. They have some decent hiking there. 2.5 weeks of hiking in Penang is quite a lot. There are a lot of trails leading up to Penang Hill as well as trails around the national park but I don't know if 2.5 weeks will keep you occupied. Unless you are planning to explore the coastal region and city as well.

I assume you are traveling to Redang or Perhentian? Both Maxis and Celcom should work, but best to check with your resort.

No.. the Tanjong Jara resort
But thanks, will message them

If the avid hikers do go to Cameron Highlands, keep an eye out for the bones of Jim Thompson. He disappeared while on a hike there months before my first (that I remember) trip to Cameron Highlands.