Regarding Tether & Lineman's Rope. They not be used interchangeably? Or better asked, can the Tether be used as a lineman's rope by throwing a carabiner on eye loop?
The most important aspect of using the tether as a lineman's rope is... how its attached to the loops? A carabiner through the tether eye and attached to the lineman loops on the saddle should be fine on one side. How are you attaching to the other loop? And what method of adjustment will you use?
And I assume that your goal is
not to eliminate carrying the 2 lines, right? Doing so would not be advised.
My method for safe climbing...
Climb with lineman attached from about the 2nd step until I reach an obstacle like a branch. Attach tether above the branch and then reposition the lineman rope and continue to climb. When I reach stand height, I reattach the tether (while still attached with the lineman) and I test the tether. Once I'm certain the tether is attached safely, I disconnect the lineman. I do that because I don't like the lineman being around the tree when I attach the platform, or ROS. IMO, the lineman in place during the act of setting a platform or ROS, is a pain in the rear. Being attached with the tether during that phase allows me to place the platform without straps, ropes, buckles, haul line, etc, getting tangled or in the way.
When the platform is safely set, I step onto it...all the while, throughout the entire climb and setting of the stand, I've been attached either with the lineman or the tether...that's the reason we shouldn't try to eliminate the lineman and just climb with a tether. Most of the time, we won't be able to stay attached to the tree with only one rope. Yes, climbing a tree with no branches it could be done but most of us want to be prepared to climb
any tree if the situation calls for it. You need 2 lines.
Which brings up another point...I like a long tether because it allows me more mobility during the act of setting or adjusting the platform. A short tether, especially around a larger tree, doesn't always have enough tag-end to allow moving off the platform, then onto steps, and then maybe down a step or 2 in order to make an adjustment without having to reconnect the lineman...which, more often than not seems to get in my way.