Hypothesis: Contessa really did kill Taylor. Then used Magic to bring her back by shoving her soul back into her into her own body before dumping her off onto the less maniacal version of Cauldron and that's why the Summoning from the Ring of Deterrence failed: because that's a Servant being Summoned by what is being misregistered as a Servant and she is currently involved in a different "Summoning System" from Chaldea's. Programming parallel processing error. She used a back door bridge by using User controls to bypass a Summoning System and accessed the Archive for "Cu Caster- Fuyuki Singularity" directly.
Unless it gets worse I'm ruling this one out but it remains an option:
Alternatively Narrative Causality is giving a glorified Background Character Joker tier Plot Armour and she accidentally found a way around the Plot's attempt to Rail Road the original Story through since it sounded like in the early Chapters like the Characters were reading off a script that they were needing to add Taylor to and the Summoning was because the Ring of Deterrence saw that it was the Background Character who was supposed to be a statistic and not the twins at the last second so it force failed the Summon and the Plot Rail Roaded the next Station of Canon due to technically succeeding at this one. So these are the Narrative Causality given Command Seals rather then the Chaldea ones.
Another Hypothesis: because of the way a Multiverse works in the Nasuverse the Entities use Magic and Contessa hijacked the Queen Administer Shard to do...
something that revived or healed Taylor... alternatively it's that she Triggered at all and because she isn't explicitly human by Magic's definition now that caused a User self contradiction Error in the Summoning System when registering Master/Servant binding Seals and to avoid a Logic Loop the Summoning System cancels the Summoning Routine. Because Singularity Servants are pre-Summoned she effectively can add them onto her... stuff... thing... the alternative is Goatia did something to Chaldea to deny Team A any Servants in the unlikelihood of survivors.
So my Four Hypotheses:
1: Plot is fighting with Narrative Causality and Taylor isn't supposed to have a Personality or Autonomy as a Background Character.
2: Entities fk'ing with Metaphysics due to encountering Worlds where the Defenses target them if they don't redesignate their Hosts as simultaneously not and as "Host Species" and "Alien Species". Oddly enough this means Taylor can kill most of the Beasts by punching them (ignoring how their other defenses would counter this) repeatedly in the face due to Authority of the Beast being given a Logic Loop Error every single solitary time she touches the thing.
3: Contessa is doing Voodoo for fun and profit and the World doesn't know how to feel about that.
4: Goatia is a fan of the "To Be Taylor is To Suffering" Meme.
*since she got Command Seals.
**since not being able to Summon yourself is stupidest thing anyone could come up with and even if it was true it still would have chosen a second closest Servant especially since a Catalyst-less Summoning is essentially throwing a net out into the Ring of Deterrence and hoping to either catch something or have a Spirit answer.
There'd been talk that it might not work on any Servant not contracted to Chaldea's FATE system
I don't see why it would, it's essentially two different instances of subtly different programs running. Document 12 in Program F isn't Document 12 in Program C even if they can access the Docu 3 version of Docu 12 Prog-F in the Archive.
A quick glance at Mash gave me nothing. That was about what I'd been expecting.
hm... figured.
When I looked at Medusa, I could see her Mystic Eyes skill, along with Monstrous Strength and Independent Action, a skill normally reserved for the Archer class. Her Class skill, however, in particular her Magic Resistance…
Must be the link to the Fuyuki Singularity via Caster. If it really is this Singularity's Grail War then I do wonder how many Singularities she will have to go through before her entire arm is just one long reddish sleeve from all the Command Seals overlapping.
Caster's surprising skill with his staff,
Shillelagh. It's an Irish weapon that's basically a "great club" as it would be known in DnD. Druids used them as walking sticks. Also in DnD there is a Druid Spell of the same name which literally is just "hit thing hard". They can also come in mace variants.
Not with the business end of her scythe.
I still have no clue why the Grail decided to Summon the Comicon Cosplay version of Medusa. There's no reason for her to be Ruby Rose but whatever. At least her being a Rider made sense from a certain perspective given in the Real World Myth she was supposed to be the mother of Pegasus. From what I can vaguely remember there is no mention of a spear or scythe or even a sickle in the Nasuverse version unless this one got twisted by the Grail into a version that stole Chronos' scythe/sickle.