Yes, I'd be concerned, too. There is never a reason to remove the tether, unless it's to replace it. I would really question whoever gave it to you--you need to know with 100% certainty that this seat was never in an accident and that the harness straps were not improperly washed or handled.
If all turns out okay, yes, you can order a new tether strap from Evenflo. You are correct--the seat you have goes to only 30 lbs rearfacing (and it's also really, really short for rearfacing use). It's almost always outgrown by height rearfacing, and definitely not before kids are ready to be turned around.
Otherwise, it was our favorite seat!
hths
henrietta