Proof of proposition 7.8.2.
To prove part ([00GH]), fix an \(n\geq 0\) and consider the functor \(S^0 = \{0, 1\} \rightarrow\mathrm{Alg}_{\mathcal O}(\mathcal P)^{\otimes}\), sending \(0\) to \((b, \ldots, b)\) and \(1\) to \((c)\). Fix a \(\mu \in \mathrm{Mul}_{\mathrm{Alg}_{\mathcal O}(\mathcal Q)}(Fb, \ldots, Fb; Fc)\). This determines a commuting square of \(\infty\)-categories 
The fiber of \(\mathrm{Mul}_{\mathrm{Alg}_{\mathcal O}(\mathcal P)}(b,\ldots, b; c) \rightarrow\mathrm{Mul}_{\mathrm{Alg}_{\mathcal O}(\mathcal Q)}(Fb,\ldots, Fb; Fc)\) at \(\mu\) is precisely the space of lifts of this square. By observation 7.8.3, this space of lifts is equivalent to the space of lifts 
By lemma 7.8.1, to prove contractibility of this space of lifts, it suffices to verify that for each \(p_0, p_1\in \mathcal P^{\otimes}\) in the image of \(\{0\} \times \mathcal O^{\otimes} \rightarrow S^0 \times \mathcal O^{\otimes} \rightarrow\mathcal P^{\otimes}\) and \(\{1\} \times \mathcal O^{\otimes} \rightarrow S^0 \times \mathcal O^{\otimes} \rightarrow\mathcal P^{\otimes}\), respectively, any square 
has a contractible space of lifts. Using the Segal condition on \(\infty\)-operads, this precisely unpacks to the condition in the statement of the proposition.
To prove part ([00GI]), fix an \(n\geq 0\) and a point \(h\in \mathrm{Map}_{\mathrm{Alg}_{\mathcal O}(\mathcal P)}(a, \ldots, a; c)\). Let 
denote the outer horn. Then, the multi-ary operation \(h\) together with our original operation \(f\in \mathrm{Mul}_{\mathrm{Alg}_{\mathcal O}(\mathcal P)}(a;b)\) assembles into a commutative diagram as on the right: 
The fiber of \(\mathrm{Mul}_{\mathrm{Alg}_{\mathcal O}(\mathcal P)}(b,\ldots, b; c) \rightarrow\mathrm{Mul}_{\mathrm{Alg}_{\mathcal O}(\mathcal P)}(a, \ldots, a; c)\) at \(h\) is precisely the space of lifts of the right square. Since the left square is a pushout, this space is equivalent to the space of lifts of the total square. By observation 7.8.3, this space of lifts is equivalent to the space of lifts of the square 
and hence to the space of lifts of the square 
By lemma 7.8.1, a sufficient condition for contractibility of this space is that for all pair of objects \(c_0, c_1 \in \mathrm{Fun}([1], \mathcal P^{\otimes})\) in the image of \(\{0\} \times \mathcal O^{\otimes} \rightarrow S^0 \times \mathcal O^{\otimes} \rightarrow\mathrm{Fun}([1], \mathcal P^{\otimes})\) and \(\{1\} \times \mathcal O^{\otimes} \rightarrow S^0 \times \mathcal O^{\otimes} \rightarrow\mathrm{Fun}([1], \mathcal P^{\otimes})\), respectively, the space of lifts of all commuting squares of the form 
is contractible. Using the Segal condition on \(\mathcal P^{\otimes}\), this is satisfied provided the conditions in the statement of the proposition hold. ◻