/-- The canonical morphism of rings from `R<x> ⧸ (I*R<x>)` to `S ⧸ (I*S)` is an isomorphism
    when `I` and `(conductor R x) ∩ R` are coprime. -/
noncomputable def quotAdjoinEquivQuotMap (hx : (conductor R x).comap (algebraMap R S) ⊔ I = ⊤)
    (h_alg : Function.Injective (algebraMap R<x> S)) :
    R<x> ⧸ I.map (algebraMap R R<x>) ≃+* S ⧸ I.map (algebraMap R S) := by
  let f : R<x> ⧸ I.map (algebraMap R R<x>) →+* S ⧸ I.map (algebraMap R S) :=
    (Ideal.Quotient.lift (I.map (algebraMap R R<x>))
      ((Ideal.Quotient.mk (I.map (algebraMap R S))).comp (algebraMap R<x> S)) (fun r hr => by
      have : algebraMap R S = (algebraMap R<x> S).comp (algebraMap R R<x>) := by ext; rfl
      rw [RingHom.comp_apply, Ideal.Quotient.eq_zero_iff_mem, this, ← Ideal.map_map]
      exact Ideal.mem_map_of_mem _ hr))
  refine RingEquiv.ofBijective f ⟨?_, ?_⟩
  · --the kernel of the map is clearly `(I * S) ∩ R<x>`. To get injectivity, we need to show that
    --this is contained in `I * R<x>`, which is the content of the previous lemma.
    refine RingHom.lift_injective_of_ker_le_ideal _ _ fun u hu => ?_
    rwa [RingHom.mem_ker, RingHom.comp_apply, Ideal.Quotient.eq_zero_iff_mem, ← Ideal.mem_comap,
      comap_map_eq_map_adjoin_of_coprime_conductor hx h_alg] at hu
  · -- Surjectivity follows from the surjectivity of the canonical map `R<x> → S ⧸ (I * S)`,
    -- which in turn follows from the fact that `I * S + (conductor R x) = S`.
    refine Ideal.Quotient.lift_surjective_of_surjective _ _ fun y => ?_
    obtain ⟨z, hz⟩ := Ideal.Quotient.mk_surjective y
    have : z ∈ conductor R x ⊔ I.map (algebraMap R S) := by
      suffices conductor R x ⊔ I.map (algebraMap R S) = ⊤ by simp only [this, Submodule.mem_top]
      rw [Ideal.eq_top_iff_one] at hx ⊢
      replace hx := Ideal.mem_map_of_mem (algebraMap R S) hx
      rw [Ideal.map_sup, RingHom.map_one] at hx
      exact (sup_le_sup
        (show ((conductor R x).comap (algebraMap R S)).map (algebraMap R S) ≤ conductor R x
          from Ideal.map_comap_le)
          (le_refl (I.map (algebraMap R S)))) hx
    rw [← Ideal.mem_quotient_iff_mem_sup, hz, Ideal.mem_map_iff_of_surjective] at this
    obtain ⟨u, hu, hu'⟩ := this
    use ⟨u, conductor_subset_adjoin hu⟩
    

Proof state:
case h
R : Type u_1
S : Type u_2
inst✝² : CommRing R
inst✝¹ : CommRing S
inst✝ : Algebra R S
x : S
I : Ideal R
hx : comap (algebraMap R S) (conductor R x) ⊔ I = ⊤
h_alg : Function.Injective ⇑(algebraMap (↥(adjoin R {x})) S)
f : ↥(adjoin R {x}) ⧸ Ideal.map (algebraMap R ↥(adjoin R {x})) I →+* S ⧸ Ideal.map (algebraMap R S) I :=
  Ideal.Quotient.lift (Ideal.map (algebraMap R ↥(adjoin R {x})) I)
    (RingHom.comp (Ideal.Quotient.mk (Ideal.map (algebraMap R S) I)) (algebraMap (↥(adjoin R {x})) S))
    (_ :
      ∀ r ∈ Ideal.map (algebraMap R ↥(adjoin R {x})) I,
        (RingHom.comp (Ideal.Quotient.mk (Ideal.map (algebraMap R S) I)) (algebraMap (↥(adjoin R {x})) S)) r = 0)
y : S ⧸ Ideal.map (algebraMap R S) I
z : S
hz : (Ideal.Quotient.mk (Ideal.map (algebraMap R S) I)) z = y
u : S
hu : u ∈ conductor R x
hu' : (Ideal.Quotient.mk (Ideal.map (algebraMap R S) I)) u = y
⊢ (RingHom.comp (Ideal.Quotient.mk (Ideal.map (algebraMap R S) I)) (algebraMap (↥(adjoin R {x})) S))
      { val := u, property := (_ : u ∈ ↑(adjoin R {x})) } =
    y
case refine_2.intro.hf
R : Type u_1
S : Type u_2
inst✝² : CommRing R
inst✝¹ : CommRing S
inst✝ : Algebra R S
x : S
I : Ideal R
hx : comap (algebraMap R S) (conductor R x) ⊔ I = ⊤
h_alg : Function.Injective ⇑(algebraMap (↥(adjoin R {x})) S)
f : ↥(adjoin R {x}) ⧸ Ideal.map (algebraMap R ↥(adjoin R {x})) I →+* S ⧸ Ideal.map (algebraMap R S) I :=
  Ideal.Quotient.lift (Ideal.map (algebraMap R ↥(adjoin R {x})) I)
    (RingHom.comp (Ideal.Quotient.mk (Ideal.map (algebraMap R S) I)) (algebraMap (↥(adjoin R {x})) S))
    (_ :
      ∀ r ∈ Ideal.map (algebraMap R ↥(adjoin R {x})) I,
        (RingHom.comp (Ideal.Quotient.mk (Ideal.map (algebraMap R S) I)) (algebraMap (↥(adjoin R {x})) S)) r = 0)
y : S ⧸ Ideal.map (algebraMap R S) I
z : S
hz : (Ideal.Quotient.mk (Ideal.map (algebraMap R S) I)) z = y
this : y ∈ Ideal.map (Ideal.Quotient.mk (Ideal.map (algebraMap R S) I)) (conductor R x)
⊢ Function.Surjective ⇑(Ideal.Quotient.mk (Ideal.map (algebraMap R S) I))

Next tactic:
simp only [← hu']