/-- The underlying morphism of the forward component of the associator isomorphism. -/
noncomputable def hom :
    ((P.tensorBimod Q).tensorBimod L).X ⟶ (P.tensorBimod (Q.tensorBimod L)).X :=
  coequalizer.desc (homAux P Q L)
    (by
      dsimp [homAux]
      refine' (cancel_epi ((tensorRight _ ⋙ tensorRight _).map (coequalizer.π _ _))).1 _
      dsimp [TensorBimod.X]
      slice_lhs 1 2 =>
        rw [← comp_tensor_id, TensorBimod.π_tensor_id_actRight, comp_tensor_id, comp_tensor_id]
      slice_lhs 3 5 => rw [π_tensor_id_preserves_coequalizer_inv_desc]
      slice_lhs 2 3 => rw [associator_naturality]
      slice_lhs 3 4 => rw [← id_tensor_comp, coequalizer.condition, id_tensor_comp, id_tensor_comp]
      slice_rhs 1 2 => rw [associator_naturality]
      slice_rhs 2 3 =>
        rw [MonoidalCategory.tensor_id, tensor_id_comp_id_tensor, ← id_tensor_comp_tensor_id]
      

Proof state:
C : Type u₁
inst✝⁴ : Category.{v₁, u₁} C
inst✝³ : MonoidalCategory C
A B : Mon_ C
M : Bimod A B
inst✝² : HasCoequalizers C
inst✝¹ : (X : C) → PreservesColimitsOfSize.{0, 0, v₁, v₁, u₁, u₁} (tensorLeft X)
inst✝ : (X : C) → PreservesColimitsOfSize.{0, 0, v₁, v₁, u₁, u₁} (tensorRight X)
R S T U : Mon_ C
P : Bimod R S
Q : Bimod S T
L : Bimod T U
⊢ ((α_ P.X Q.X T.X).hom ⊗ 𝟙 L.X) ≫
      (α_ P.X (Q.X ⊗ T.X) L.X).hom ≫
        (((𝟙 P.X ⊗ (α_ Q.X T.X L.X).hom) ≫ (𝟙 P.X ⊗ 𝟙 Q.X ⊗ L.actLeft)) ≫
            (𝟙 P.X ⊗ coequalizer.π (Q.actRight ⊗ 𝟙 L.X) ((α_ Q.X T.X L.X).hom ≫ (𝟙 Q.X ⊗ L.actLeft)))) ≫
          coequalizer.π (P.actRight ⊗ 𝟙 (coequalizer (Q.actRight ⊗ 𝟙 L.X) ((α_ Q.X T.X L.X).hom ≫ (𝟙 Q.X ⊗ L.actLeft))))
            ((α_ P.X S.X (coequalizer (Q.actRight ⊗ 𝟙 L.X) ((α_ Q.X T.X L.X).hom ≫ (𝟙 Q.X ⊗ L.actLeft)))).hom ≫
              (𝟙 P.X ⊗ TensorBimod.actLeft Q L)) =
    (α_ (P.X ⊗ Q.X) T.X L.X).hom ≫
      (((𝟙 (P.X ⊗ Q.X) ⊗ L.actLeft) ≫
            (coequalizer.π (P.actRight ⊗ 𝟙 Q.X) ((α_ P.X S.X Q.X).hom ≫ (𝟙 P.X ⊗ Q.actLeft)) ⊗ 𝟙 L.X)) ≫
          (PreservesCoequalizer.iso (tensorRight L.X) (P.actRight ⊗ 𝟙 Q.X)
              ((α_ P.X S.X Q.X).hom ≫ (𝟙 P.X ⊗ Q.actLeft))).inv) ≫
        coequalizer.desc
          ((α_ P.X Q.X L.X).hom ≫
            (𝟙 P.X ⊗ coequalizer.π (Q.actRight ⊗ 𝟙 L.X) ((α_ Q.X T.X L.X).hom ≫ (𝟙 Q.X ⊗ L.actLeft))) ≫
              coequalizer.π
                (P.actRight ⊗ 𝟙 (coequalizer (Q.actRight ⊗ 𝟙 L.X) ((α_ Q.X T.X L.X).hom ≫ (𝟙 Q.X ⊗ L.actLeft))))
                ((α_ P.X S.X (coequalizer (Q.actRight ⊗ 𝟙 L.X) ((α_ Q.X T.X L.X).hom ≫ (𝟙 Q.X ⊗ L.actLeft)))).hom ≫
                  (𝟙 P.X ⊗ TensorBimod.actLeft Q L)))
          (_ :
            (tensorRight L.X).map (P.actRight ⊗ 𝟙 Q.X) ≫
                (α_ P.X Q.X L.X).hom ≫
                  (𝟙 P.X ⊗ coequalizer.π (Q.actRight ⊗ 𝟙 L.X) ((α_ Q.X T.X L.X).hom ≫ (𝟙 Q.X ⊗ L.actLeft))) ≫
                    coequalizer.π (P.actRight ⊗ 𝟙 (tensorBimod Q L).X)
                      ((α_ P.X S.X (tensorBimod Q L).X).hom ≫ (𝟙 P.X ⊗ (tensorBimod Q L).actLeft)) =
              (tensorRight L.X).map ((α_ P.X S.X Q.X).hom ≫ (𝟙 P.X ⊗ Q.actLeft)) ≫
                (α_ P.X Q.X L.X).hom ≫
                  (𝟙 P.X ⊗ coequalizer.π (Q.actRight ⊗ 𝟙 L.X) ((α_ Q.X T.X L.X).hom ≫ (𝟙 Q.X ⊗ L.actLeft))) ≫
                    coequalizer.π (P.actRight ⊗ 𝟙 (tensorBimod Q L).X)
                      ((α_ P.X S.X (tensorBimod Q L).X).hom ≫ (𝟙 P.X ⊗ (tensorBimod Q L).actLeft)))

Next tactic:
slice_rhs 3 5 => rw [π_tensor_id_preserves_coequalizer_inv_desc]