I've had sheep meat hung from 0 to 9 days all been fine.
I *have* experienced poor butchering making meat tough. I don't know the mechanisms but I think it is a thing.
the lamb is tough more than anything else. it cooks up like mutton!
Mutton isn't tough if cooked moist, long and slow/low. Maybe worth trying?
Lamb/hogget meat that's eaten lots of cake does have a different texture to grass-fed, and considerably less flavour. But I haven't experienced it making meat tough.
Stress can wreck meat, 100%. Both texture and taste. From the gathering, being kept in overnight (if they are), handling at sorting and loading, the travel, what happened at the abattoir overnight, everything. We work hard to minimise all stresses and generally get awesome meat. We do sometimes take them to the abattoir a day early, and ask for them to be penned together and not mixed with others if possible. It's a small rural abattoir and usually a very nice, gentle atmosphere pervades.
Years ago, ex-BH kept getting some loin and leg meat condemned. I looked into possible causes, and realised the lambs were climbing over each other in a narrow offshoot in the overnight pen. We switched things around so they'd overnight in a larger, square pen, and we took our time letting them potter through the race to the loading area so they didn't jumble in the race either, and only ever had one condemned leg after that. (Which we think might have been a dog gripping a recalcitrant lamb as we gathered them.)
You may well be the very best transporter of livestock in the land, but just in case... I see a lot of newbies / inexperienced folks driving a trailer full of livestock the same as they'd drive with a load of hay. The livestock will be stressed and bruised to hell if driven like that. I ask new-to-it drivers to imagine they have a bucket full to the brim of slurry in the passenger footwell and don't want it slopping over the side

. No sudden changes at all, decelerate don't brake, oh so gentle around bends, long slow acceleration, etc.
I have to say that in this case, my money's on stress (quite possibly before arrival at the abattoir), or that not being your meat. My predecessor here always slap marked all the pigs on both shoulders as she was sure she'd had someone else's pork once.