/-- 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]
      

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)) =
    ((coequalizer.π (P.actRight ⊗ 𝟙 Q.X) ((α_ P.X S.X Q.X).hom ≫ (𝟙 P.X ⊗ Q.actLeft)) ⊗ 𝟙 T.X) ⊗ 𝟙 L.X) ≫
      ((α_ (coequalizer (P.actRight ⊗ 𝟙 Q.X) ((α_ P.X S.X Q.X).hom ≫ (𝟙 P.X ⊗ Q.actLeft))) T.X L.X).hom ≫
          (𝟙 (coequalizer (P.actRight ⊗ 𝟙 Q.X) ((α_ P.X S.X Q.X).hom ≫ (𝟙 P.X ⊗ Q.actLeft))) ⊗ L.actLeft)) ≫
        (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 1 2 => rw [associator_naturality]