Technically yes it can be fatal. It depends on the dose, the tricyclic or tetracyclic AD in question, and the individual's physiological factors (size, age, gender, personal enzyme profile, diet, experience, etc.). The main thing is that the combination could lead to a synergistic effect on adrenaline and serotonin caused by the AD drug being a monoamine reuptake inhibitor of those two neurotransmitters while LSD is a releasing agent for them, and other neurotransmitters, too, of course. But the point is: the combo could cause serotonin syndrome, which could cause a seizure/convulsions and that could be fatal… While that doesn't normally happen w/serotonin syndrome, it's still very unpleasant and generally sucks shit. I'm not a medical professional, so don't take any of this as advice; ask your doctor before any experiments, but I think that's the main concern you might be asking about here.
Yes, they are similar and share many, though not all, of the same traits, qualities, contraindications, etc. Use caution and talk to a professional if you're uncertain.
Again: I'm not a medical professional and this is just general facts and some guesswork. My hunch is: no, you're not letting the peaks of each drug overlap each other and I think mostly the Mirtazipine is going to interfere with the trip / make it weaker than it would be otherwise. I would skip the dose the night before. After the trip, I would take the Mirtazipine per usual and sleep the sleep of the Gods, and wake up feeling refreshed and revitalized. But that's just a hunch.
Yeah but still, I think the idea the site admins have is to organize all the random data bits like this into taxonomically correct filing via threads. But eh, that's what search engines are good for, right? I mean, Yahoo, at one point, was attempting to index the entire Internet sometime circa, idk, 1997-ish…Pretty soon it became impossible and in came Google with an amazing, new search algorithm that beat the brakes off all the other search engines at the time. Yahoo abandoned the indexing and tried catching up, but it was too late. The point here is: coo-coo cachoo.