When you have things like, Medusa qualifies for Lancer because her head got cut off by a polearm! Or, how Parvati is a Lancer because her husband has a spear. Or, Shakespear qualifies for Caster because what else would he qualify for, never mind that he has 0 access to actual magic or magecraft. Even without event weirdness the qualifications for each class are low.
To be fair, it's not that Medusa qualifies as a Lancer because she was killed by Harpe - but rather, she was
summoned as a Lancer because Gorgon was in the Singularity and the combination of the younger Medusa and Harpe was needed to take her down.
It's worth remembering that though Perseus was indeed the slayer of Medusa and the original wielder of Harpe, he only actually did it through guile tactics and so while Harpe could kill Gorgon, Perseus would still find it difficult to get close especially with Tiamat's Authorities strengthening Gorgon. But Ana? Ana had the advantage where she was Gorgon's blind spot in that Gorgon had difficulties sensing her, and she could also take over Gorgon when given the opportunity. Thus, sensing this, the Counter Force decided to summon a combination of Ana and Harpe to slay Gorgon, and as a result Medusa Lancer came to be.
As for Lancer Parvati, it's also worth remembering that Parvati herself was also an odd scenario to begin with. It was implied when she first came out that she manifested as a Sakuraface for a reason unknown to her during her debut event, and later we only found out that this was foreshadowing the Ooku Event and Kama/Mara's Beastification. To slay Kama/Mara, one needed Shiva, but Ooku placed a lot of disadvantages on males, so instead it was Parvati who was summoned with one of his weapons as a Lancer. In the process, Parvati also ended up carrying the spirit of Kasuga whose identity was crucial in taking down Kama in one fell swoop. So generally, both Servants fall under the category of "summoned as specific counters to major threats by the Counter Force" and you can see this happening for several more Servants.
Lastly, authors as Casters sounds like an odd thing, yes, but the justification there is that words and stories are ancient forms of magic on their own. With how legends are made through the written and the oral works of writers and bards, the mere act of creating and telling stories itself is a form of legend - but as you might surmise, this generally ends up with some rather specialized power sets for these author Casters. Dumas makes fakes, Andersen understands humans and aids in their development. Shakespeare retells legends and tragedies, while Scheherezade is the storyteller that defeats evil Kings.
Are these justifications weird and often times very arbitrary? Yes, they are. But they certainly have more internal sense than King Arthur slapping Excalibur on to a water gun and qualifying for an Archer class that way.
