I'll try to answer to all of this.

"Fusion" is when different identities fuse and recognize each-other as the same "me". You can have parts fusing together without being completely fused (aka Final Fusion).
Functional multiplicity is when you have at least two different identities, and all the traumas are healed, all the inner ressources are shared.
No fusion is 100% permanent. Stress can make one un-fuse or create new parts/identities. If one wants to keep fully fused, it requires life-long work to keep everything in order inside. Most of it becomes a habit after a while though.
In functional multiplicity, you can talk to the other parts, insiders, alters etc. But you can also have direct access to their thoughts and abilities, without the need to switch or have inner dialogue.
I experience fusion as "being blended all the time". So it kinda creates a "new" collective identity for the fused parts. But in the same time, it's like being all of them at the same time equally.
I don't know if it's specific to me or if it's because I'm fused "only on the front" (meaning, not completely 100% fused) but I still experience "inner messages" and "inner thoughts" as coming from specific parts when I really need to become aware of something. It feels a bit as if I was looking closely at my foot to check if eveything is alright here. Or as if I bumped my little toe in the table, and suddently became aware that yep, little toe exists and hurts bad. It's still me, but it's also "I am super aware this specific part of me exists". As an example, I got some annoying interactions piling up with someone so me-Zami made themself be known annoyingly (headache, pressure in the head etc.) and with pseudo-hallucinations of them stomping their foot on the ground, until I agreeded to "listen" to what they had to say i.e. pay more attention to specific thoughts and emotions.
A friend of mine describes this as being "a main host with passive influence from the other parts and a high level of integration". I call this "being fused at the front, more dissociated in the back". Potato potAto I guess.
I can also "summon" specific, already fused/integrated parts to talk with them if I really need this specific kind of interaction in order to think something through. If I really need to know specifically about how I function inside, I "summon" Theia because she is kind of the "inner secretary / gatekeeper". But it feels like looking closely at my little toe and talking to my own toe, rather than talking to "not-me" like it used to feel.
I also read in various places, including stuff from DID therapists, that it's best to let the system sort the fusions of identities by itself. Focusing on integration (sharing ressources, healing traumas, connecting to the here and now...) and let the fusion-or-not-fusion just happen. Not forcing it one way nor the other.
If you need it, it will happen. If you need to stay as many alters, this will remain so.
My girlfriend's identities are gravitating more and more towards being human and she is also gravitating towards "functional multiplicity". My own identities remain varied in nature (angel, qilin, dryad, fictive characters...) and I just am all of this blended together and gravitating towards final fusion while still being able to talk to my various parts if I need to. It just feels like talking to myself in a mirror or to my own hand rather than talking to someone else. So I guess there is no "one size fits all" rule about this.
Your system does what it needs. If you need to be able to talk to your twin Juno and spirit guides and so on, this mechanism will be kept. Does not matter how integrated you are, if you need to hear them and see them, they'll stay separate enough so you can hear and see them. The conversations might be faster though, more like direct download of thoughts rather than hearing long speeches. The efficiency of communication increases with functional multiplicity compared to having less integration.
I hope I answered all your questions and did not miss any!