You don't want to use cheap shift cables like that. If you want to save money just replace cables and housing on an as-needed basis which is

- Less frequent for the left shifter than right
- Less frequent for the housing than the cable

Note that you can secure the cable with something like a bottle cage bolt and lock your rear derailleur into a more favorable gear.

Also note that cables usually don't fail without warning. Once the first strands fray shifting to smaller cogs gets a little sluggish. While you might get acceptable performance by reducing cable tension, you really want to look into the root cause because it could be a bent hanger that also gets your rear derailleur too close to the spokes.

Irrelevant. Cable and housing life are functions of how much you shift.

I shift like I have ADHD and usually fray rear cable strands within 4000 miles which is less than a year. One (Campagnolo like I usually use) cable only made it 2000 miles for some reason.