Hi and welcome to the forum!

More to the point, I was just a tad bit confused about the ending of the Millennium Hawk Arc/this chapter and was hoping someone could help enlighten me.
It's the Millennium
Falcon arc (it's a Star Wars pun). The chapter that ended with the arc is the Chapter of Falconia, and this thread is about
episode 304. Just so we're clear about the terminology.

I get that Skull Knight attacks Femto with his dimensional sword (I hear Sword of Resonance isn't the correct translation which is what I was accustomed to calling it
)
You can just call it a "beherit sword", which is essentially what it is. "Sword of Resonance" is indeed a mistranslation.
And since you're a big fan of Berserk, I recommend you to buy the manga instead of just reading online scans. Not only will you find less translation mistakes in there, but you will be officially supporting Berserk's author.
Dark Horse, the official editor of Berserk in the USA, sees so little demand for it that reprinting the older volumes is almost seen as a gamble for them. And yet there are more English-speaking fans than ever before! That's just not right. Without real support from the fans, the risk is that someday support will be dropped altogether. I think that would be very sad.
merges the Astral with the Ideal world (basically bringing about "Fantasia").
Actually the astral world merged with the corporeal world.
how was Ganishka's death/SK's strike able to trigger the merging of realms/worlds as it did?
After what happened in Vritannis, Ganishka knew he just couldn't stand against Griffith. Aside from his army of apostles, Griffith himself just was too strong, and had such an influence over Ganishka that he could bring him to his knees without effort. Ganishka had to become more powerful.
Desperate, with no contingency plan but to make a last stand, Ganishka decided to plunge himself into the "artificial beherit" he and Daiba had created. That contraption used captured apostles to create a portal to the depths of the astral world. They had been using it to create the Daka. So Ganishka went down the "vat", descended at the bottom of the astral world, and absorbed as much power as he could.
The experience changed him. He was transmuted again, not just an apostle anymore but something more. Much, much more. He had tried to gather enough power to match that of a member of the God Hand, and so his corporeal form (his physical body) wasn't enough for it. If you'll remember, members of the God Hand cannot normally materialize themselves in the corporeal world. That's because they have way too much power. When Femto came into the world, incarnated into a new Griffith, it was a "once in a thousand years" event that had likely been carefully planned. Ganishka's self-made method ended up with him having that gigantic body, to contain an equally gigantic power. But beyond his body, Ganishka himself couldn't quite handle all that power. He became half mad.
Anyway, for all his efforts, he was still no match for a member of the God Hand. He had brought back a mind blowing amount of power, but Femto pacified him with ease. Then came the Skull Knight. Always ready to strike at every tenuous opportunity. So much so that this time his attempt was expected. He struck, but Femto deflected his blade. And it wasn't just any blade. The Skull Knight had been collecting beherits for a long time, storing them inside his body. He thought the resulting weapon would be potent enough to deal with the God Hand.
But what does the sword do? It cuts through the layers of the world. It does so thanks of the beherits it uses. In volume 13, the beherits are called "Ikai e no Yobimizu". That roughly translates as "drops of primed water leading to another world". They come from the deepest depths of the astral world, the abyss (as we can see during the Eclipse). When they are activated, they create a link between the corporeal world and their place of origin. By coating his weapon with them, the Skull Knight turns it into a "Yobimizu no Tsurugi". A sword that can "prime a pump". Unfortunately the meaning of the expression does not translate well into English, which is why we just say "beherit sword".
And now, for your actual question: when SK's sword struck Ganishka, it cut right through him. But it didn't just pierce his skin like a normal sword would. It cut through him on the astral level. Like an overblown balloon that tears in the middle, Ganishka was ripped inside out. All of the vast power that he had brought back into the world and had tried to contain within himself then spilled over the corporeal world, covering it integrally in one big wave of light. Thus were the worlds merged.
It's worth noting that this had been the God Hand's plan all along. It went off without a hitch.