And of course the Death Star can be destroyed or at least stopped with sufficient firepower. However, even another Death Star is unlikey to survive a hit by a Death Star blast (given that it can blow up shielded planets). Hitting the dish is certainly a good way of disabling it's strongest weapon, but it still carries a lot of fighters and is armed to the teeth with other weapons, while being a substantial base for enemy operations even of the superlaser is disabled.
There is also the fact that the there is a good chance the Death Star's shields are too strong for the Eclipse to penetrate, given that it is stated to only be able to sear continents which would be below the minimum energy required to penetrate planetary shields as well as presumably the Death Star's shields.
The distance between Alderaan and DS during the planet's destruction is often stated to be around 100 000 km. With most of the 10^38 joules released into space, it should have been impacted with something like 2*10^31 joules - without anyone on the station so much as knocked down. I would say the Eclipse penetrating the shields is, indeed, no guarantee.
The problem is that the minimum energy required to destroy a planet is much less than the energy required to penetrate planetary shields given that Alderaan's shield was able to resist the Death Star's blast for a fraction of a second.
Not certain. SW beam weapons seem to have an invisible component, which is sometimes separate. In Alderaan's case, there was a secondary explosion, probably from that very component (the first explosion, for all we know, could be just the upper levels vaporizing) - and if we don't assume the invisible part carried most of the energy, we should assume (from the percentage
of the visible beam absorbed) that the DS wasn't built with so much as a 100% overkill against existing
shields - and no one will design an Ultimate Weapon that way. It must, after all, be capable of defeating not only existing shields, but any possible upgrades caused by its threat.