You've set up a beautiful quandary and definitely want to keep the quandary, but I think you would ruin it if you work out some easy way for Tate to destroy the stone without endangering the home front. In fact, if you could do it that way, it might actually feel a little cheap to the readers. What you've accomplished is a much better dilemma.
Frankly, I'm pretty sure the first thought on Marek's mind would be "Let's do both!" and destroy Ev's forces, confident with all the standard hubris of a great antagonist that he can do so and still beat Tate to the stone.
Applying Sun-Tzu style strategy to this (which is a great technique; if you have not yet read
The Art Of War, then fix that!), I would have said the stone is not the issue, the castle is, so you need to use Marek's vulnerability to trick him into thinking he needs to get back and defend the stone, while Tate stays and takes advantage of his absence.
However, that runs afoul of your story plan, so let's get a little more creative. Yes the castle is the issue, but we can assume maybe my first plan runs the serious risk of Marek realizing his error and getting back before Tate can finish wreaking havoc on the hapless defenders. So, instead we need to find an even better subterfuge (Sun-Tzu never attacks frontally if he can help it. Everything in his craft is about misdirection, just like a stage magician.)
So, you need a trick that allows Marek to think he is still in a standoff with Tate long enough for our nascent lovebirds to make it all the way to the castle.
However, you also need Marek to believe he can't take the time to rain death and destruction on Ev's forces before getting back. I'm assuming from your statement about 'recharging' that his power doesn't go away upon the stone's destruction. So, if it is already destroyed, he might use what is left to take his vengeance out on those he can reach. He needs to believe he only has enough time to get back.
So, you need to make sure that Marek believes he can save his stone. (BTW, I also assume it isn't portable, or he would keep it close.) Comes back to rescue it and while he is absent, Ev's forces can have fun storming the castle.
I really think that what you want here is for Ev and Tate to split their command at this point. Ev is back to lead the charge (and maybe Ev's presence via some charm or working of Tate's magic makes the difference in the battle) whilst Tate is at the stone, ready to destroy it and Marek together.
I think Sun-Tzu would approve of that plan. And it gives you an opportunity for a bittersweet parting and final reunion as a bonus.
...
Regards,
Eric Fretheim
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