Going to the Forest of Shadows to go kill a necromancer within his own home base (which we know is the location of a nexus), to steal his Big Book of Bad Magic... is definitely something we can do.
...I'm actually totally behind the idea, but I'm phrasing it like that to highlight the danger. I think we'd see very decent support from the idea if we use the Gambler (or the Father) while doing so, and maybe bring Johann and/or Egrimm as backup. Mathilde's a lot stronger as a wizard and as a swordswoman than she was when she took down Alkharad's college... But he still very nearly killed her. A certain level of caution is warranted.
And depending on what canon Boney's using, the master of the Tower of Melkhior is separated somewhere between one and three degrees from Nagash. Canon originally is that W'soran got ganked by his apprentice, Melkhior the Ancient, who drained him dry of blood and then ate him. Several thousand years later, Melkhior got ganked by his apprentice, Zacharias the Everliving (though in canon he does that around 2506 IC). One accounting of canon says that W'soran transmitted his spirit to Melkhior, though, so... shrug. Who knows which interpretation Boney is going for.
...Well, regardless of danger to actually assaulting the Tower, the first step is to merely scout it out from a distance, alongside the other Forest of Shadows nexuses. That part we're definitely doing no matter what, I imagine, and I will continue to support the notion of doing so as part of a Father-plan. Haletha is the goddess of Protection from the Forest of Shadows, so who knows - perhaps she'll look kindly upon us and give us a greater narrative benefit than the Gambler's. We'd be acting directly in Her interests, after all.
I agree that it is dangerous, I would argue too dangerous for us to do now. It is a fun idea to be certain, and actually one that definitely needs considering. I have three reasons outside of the ones you have listed and one hypothesis that makes me want to do something else first.
1. Best case is Zacharias and his Dragon. (assuming he has it at this point) Worst case is the standard bevy of necromantic and vampire servants with what could most likely be called an army.
2. It's in the middle of the forest of shadows. Zacharias is not the only player in that neck of the woods. Orcs, beastmen and more hang around there, not to mention quite close to the north. Any army would lose half it's numbers before the walls and any strike group needs to balance at least a half dozen combatants against things known to be skilled.
3. It's a solid strike (especially with our advantages against dhar) but it would require a lot of people knowing the whole we read the Liber Mortis enough to challenge a vampire in his home lair powered by a nexus.
Onto hypothesis. I would like to do the Elven adventure in Nagarythe first to learn more about the way they work Ulgu and their viewpoint on gods. During the bringing of the Hedgewise in and Haletha was mentioned as "stealing the shadows from the forest of shadows for us." or something to that effect.
A viewpoint on gods and magic from the Elves. A local goddess who has a myth related to stealing shadows. A grey wizard who worship's a god of thieves who seems to be related to the local goddess... Oh how I love some thematic vibes that scream pick me!