Why hold'claim Osgiliath?

[William Cloud Hicklin]

Qempel, I think we can assume that Gondor tried to beef up the landing-zone defenses as best she could (although Denethor wasted manpower and time on the Rammas)- and apparently they held for two days; but with an enemy of overwhelming numerical superiority (as well as the attested morale-effect of the Black Captain), not to mention the fatigue of unrelieved troops in continuous combat- they just weren't capable of holding. There is no such thing as an impregnable position given sufficient men (and sufficient disregard for casualties). The idea that a few hundred Men of Gondor could prevent indefinitely the Morgul-Host from effecting a crossing is a non-starter.

Rommel did his level best to make Normandy 'impregnable,' remember?

why couldn't Sauron just as easily forge a new ring to rule over the one ring?

Because it wasn't 'easy' the first time, in fact Sauron invested 'the better part' (i.e. most) of his native power in the One. He hadn't sufficient left to make an even mightier one. Besides, in Tolkienverse some works, like the Trees and the Silmarils, can't be repeated.

Plus the wizards had knowledge of black-powder, I am pretty sure black-powder bombs or guns might just make it easier to win the war.

"The wizards?" Say rather Saruman. If indeed the 'fire of Orthanc' at Helm's Deep in fact was gunpowder, which we don't know.