Shooting the shorter .458Win Mag is OK in the .458Lott as they are chambered with a tapered leade in formt of the throat & are meant to allow the Win round. Tests show little if any drop in velocity, and excellent accuracy with Win Mag in .458Lott's. It is not recommened for constant use, only in an emergency.
; The .458 headspaces on the belt.
: Shooting too many Wins., may errode the chamber ahead of the shorter case.
: Some of your other examples are normal practice for making brass for the Improved chamber.ie: .375H&H in .375 WTBY or .22 Hornet in the "K" chamber. The parent case in those guns is the same length as the chamber so NO damage will result - they are designed for just that practise. The Hornet headspaces on the rim, not the shoulder & no harm results as the shoulder blows forward when fired. The .375H&H headspaces on the belt, same as the .458's. Your other example is a NO/NO - you cannot fire .30/06 safely in a .30/06GIBBS as the shoudler of this round is the headspace adjustment, and the Gibbs is further forward than the '06 case. In order to safely shoot '06's in the Gibbs chamber, the neck should be enlarged to .8mm (mnimum) or .338, then necked back down to .30 forming a "False" shoulder for a crush fit in the chamber. That 'new' shoulder will hold the case properly and safely, IF properly located. Merely seating the bullet out to contact the throat might not work due to primer blow pushing the bullet back into the case, thus developing excessive headspace upon discharge. This type handloading is not for those who aren't avid shooters of wildcat or improved rounds who have the necessary understanding of headspace and how to properly obtain it.