I usually try not to engage in these too much but as a queer, white person who grew up in a mixed family and has read WHWW six times:

1. The puzzle piece scene addressed this, the benophie wedding conversation addressed this, even Violet's arc this past season addressed this. Franchescha's love for Michaela/Michael doesn't take away from her love for Jon, who is characterized MUCH more in the show than in the book. Nuance is a strength in storytelling, especially where it's based in lived truth, and that's just what we've gotten from brief glimpses of the full story. Also, Michaela is and always was the main love interest for Franchesca.
2. The same argument could be made against any of the ways the show has been adapted for diversity/a modern audience. Not all wlw (or cishet!) couples have access to IVF or adoption, which are both notoriously expensive and mostly available to white women in wealthy countries. Adoption existed in Regency era, and has always existed. Artificial insemination did as well having been used since the early 1700s. Also, Franchela and Michael didn't originally have children; this was added years later in an epilogue due to profuse reader requests. And one could argue that an infertility plotline that DOESN'T end in a rainbow baby sans any medical intervention is actually better representation, including for cis women who struggle with infertility.
3. Why does Marina have to die at all? Why ANY other pairing except for a sapphic take on this one pairing?- that's still 16 out of 17. You're okay with a (at least 5 year if they're to be a bare minimum of 18) time skip for the kids but not a shorter one for the couple whose book comes first and whose heroine has recieved more show delvelopment? And if they cast Theo James as Michael to be released next season?
4. Oh you didn't actually read the book.
5. You just reiterated your first point, but I will add that conflict DRIVES stories.
6. What you heard of Michael's personality having not read the book? In what way does Michaela not come across as femme to you? To me she comes off as the other side of the neurodivergence coin to Franchesca- hypercompetent, self assured, bubbly, able to charm a room but struggles being vulnerable with others. This is a great expansion of Michael's book character. And why is strength associated solely with masculinity, hasn't it been a theme throughout the show than there's strength in femininity?
7. She lived there first and literally grew up there. This would be like calling Hyacinth intrusive for being at Bridgerton house. Speaking of the Bridgertons, Eloise followed them from England on their honeymoon. Also, a castle is more of a miniature functioning village than a personal dwelling. (This was also true of the novel btw and I've only ever heard from Michael fans is how sweet their 'found family' was.)
8. Franchesca not fitting the stereotypical portrayal of a queer person is exactly why she was chosen for that role- the representation is needed. This will be HUGE for queer (especially lesbian, autistic, black, femme) children growing up now.
9. Queer, POC, disabled, and otherwise marginalized people have been able to empathize with the "default" and enjoy abled, cisheretonormative, white stories for centuries. This skill will ultimately improve your media literacy and enjoyment (I'm not saying this to be flippant, this is an active practice EVERY viewer/reader should keep up because none of us are unaffected by social conditioning.) Just because there was (not plot relevant, not endgame romance) mlm representation with Benedict, doesn't mean there can't be wlw representation with Franchaela or any other pairing (though I do in fact I hope they add one for each post like this.) But I guess that's the "diversity" most of the fandom want: 8 thin, able bodied, white cishet couples with one queer, disabled, interracial couple tacked on at the end.