Tieve was never a goddess. She was a mortal reincarnation of Morrighan that slowly was pulled back to Tir Na Nog in order to transform back into Morrighan. However, she had developed a separate identity than Morrighan so when she turned into her, it was said their "souls" were fighting over the body and Morrighan's won. That's not to say that Tieve isn't still in Morrighan somewhere, but it's possible for her to be completely destroyed one day; however, Morrighan's docile, peaceful look in Mabi is very, very similar to Tieve's personality, so either Morrighan is copying her to seem innocent or they're still fighting over their body (which makes sense as during Alch it shows Morrighan being "awakened" as the goddess of destruction, different from her personality up to that point).
It's likely that Mari merged with Dana, as Dana is a central deity that as-of-yet has not been revealed in-game; Tieve is not a deity, never was, never will be.
If Morrighan really destroyed Tieve completely, which I doubt, there's no way Cichol would befriend her. Unlike Morrighan taking over Tieve's body, Cichol is really just Keaghan having achieved godhood, meaning that Cichol === Keaghan forever, and Keaghan absolutely hates Morrighan for taking Tieve from him. I can only see him working together with her if Tieve wants it.
I disagree with you on all accounts for Lugh/Morgant and Triona. In Live it's revealed that Macha somehow is already resurrected despite the ritual to summon her through Triona failing in G3, BUT Triona "died" in Shakespeare giving Lugh/Morgant a legit reason to want to destroy the gods, which is what the whole Tara incident was about.
In my headcanon, the residents of Emain Macha - Lugh included - are stuck in an ageless dreamstate where they don't age, the Tragedy actually happened long ago, the Three Warriors were all there (Mari was escaping when Merlin's spell went wrong and "froze" the inhabitants), and over time the events that people refer to as happening long ago occur with the characters involved remaining at the same age. If so, it's possible for Macha's curse to have happened right before that with Lugh being her lover that, due to his status, was politically forced to marry another woman, making Triona Macha's daughter and therefore it making sense for her to be used as a medium for Macha since demigods have been shown in Mabi to be good conductors.
Some people may have detected otherworldly power from Triona and thought she was a Fomor, especially since Macha's curse and the Tragedy both could have made Lugh defect from Humanity and the Gods which would give him a reason to become Morgant, especially since he wanted to protect his daughter. This would also give him a REASON for risking his daughter's life for Macha if he was still in love with her and wanted to bring her back.
This is why my headcanon remake of G16 etc involves Triona successfully becoming Macha only for Macha to realize that it was her daughter that's her vessel and murdering Lugh for trying to sacrifice her, and she "kills" herself to die with Lugh but Triona inherits the body and officially ascends to godhood in place of her mother. Happy ending for her since she can be with Ruari, and Lugh/Macha's starcrossed tale ends with its proper Romeo/Juliet style fate.
Extra Info: My vision of a remade Chapter 4 just consists of four plays, one for each guardian (Hamlet>Juliet>Midsummer>Golvan). G13 is really just replaced by the entire Chapter 4. The last guardian, a concept abandoned by DevCat, has to be made original (they're based on tarots, so maybe a Chariot with four beasts). My idea is for Merrow to be Bella in disguise and her play, For Golvan, to be last - it's a mysterious play that Shakespeare didn't write but was delivered to the actors and he likes it so it's played; it's basically Bella's attempt to get Shakespeare prepared for future events (Chapter 5, which consists of Macbeth, Venice, Tempest, and Pericles, not in order) with Golvan representing Shakespeare and Aisling representing Bella; Aisling is supposedly from a "play" where she's a guard of the Queen of Swords (Morrighan) and sacrifices herself to save those she loves (which is true as Bella in Live dies and believes her actions were to stop Erinn from being destroyed), meaning that Golvan/Ailsing ARE Shakespeare/Bella and she's trying to warn him...with For Golvan really meaning For Shakespeare, because it's for Shakespeare!