Withstanding Sauron

[Maéglin]

Well the exact quote is

he was robbed now of that shape in which he had wrought so great an evil, so he could never again appear fair in the eyes of men

This does not say anything at all about losing his abilities to change shape. Just that he couldnt take a fair form in order to deceive men ever again. Which of course does not say anything about him not being able to shape-shift, just that he cannot shape-shift into Annatar or other fair incarnations.

If you are going by behaviour then it is not Sauron's behaviour to be defeated valiantly and not retreat, killing his two greatest foes of the Second age. He has a history of cowardice, the duel with Huan at Tol-in-Gaurhoth, yielding to Ar-Pharazon to work his way into the heart of Numenor and feinting his defeat at Dol-Guldur in the Third Age.

You say that it is not like Sauron to go out and fight beside his troops. You are perhaps correct but the siege lasted seven years before Sauron decide to come forth into combat. It makes sense that he wanted to end the stalemate and not to mention that Sauron did do tremendous hurt to the Last Alliance before his defeat so that he could still prevail

suffered great loss by fire and by the darts and bolts of the enemy, and Sauron sent many sorties against them.

I really don't think that Sauron was just going

to run out and give Gil and Elendil a big hug before he was physically destroyed