Limits of a product
$$\left(1+\frac {j}{n^2}\right)\left(1+\frac {n-j}{n^2}\right)=1+\frac{1}{n}+\frac{j(n-j)}{n^4}$$
By AM-GM we have $$\sqrt{j(n-j)}\leq \frac{n}{2} $$ and hence $$1+\frac{1}{n} \leq \left(1+\frac {j}{n^2}\right)\left(1+\frac {n-j}{n^2}\right) \leq 1+\frac{1}{n}+\frac{1}{4n^2}$$
Therefore $$\left(1+\frac{1}{n}\right)^{\frac{n-1}{2}} \leq P[n] \leq \left( 1+\frac{1}{n}+\frac{1}{4n^2} \right)^\frac{n}{2}$$
Now $$\lim_n \left(1+\frac{1}{n}\right)^{\frac{n-1}{2}}=\left( \left(1+\frac{1}{n}\right)^{n} \right)^{\frac{n-1}{2n}}=\sqrt{e}$$ And $$\lim_n \left( 1+\frac{1}{n}+\frac{1}{4n^2} \right)^\frac{n}{2} =\lim_n \left(\left( 1+\frac{4n+1}{4n^2} \right)^\frac{4n^2}{4n+1}\right)^\frac{4n+1}{8n} =\sqrt{e}$$
Therefore $$\lim_n P[n]=\sqrt{e}$$
Considering $$P_n=\prod_{i=1}^{n-1} \left(1+\frac i {n^2} \right)\implies \log(P_n)=\sum_{i=1}^{n-1} \log\left(1+\frac i {n^2} \right)$$ Since $n$ is large, by Taylor (or even using simple equivalents) $$\log\left(1+\frac i {n^2} \right)=\frac{i}{n^2}-\frac{i^2}{2 n^4}+O\left(\frac{1}{n^6}\right)$$ making $$\log(P_n)=\frac{(n-1) \left(6 n^2-2 n+1\right)}{12 n^3}+\cdots=\frac{1}{2}-\frac{2}{3 n}+O\left(\frac{1}{n^2}\right)$$ showing a limit of $\frac 12$. Then $P_n \to \sqrt e$.
Probably too early, but added for your curiosity, using Pochhammer symbols $$P_n=\frac{\left(n^2+1\right){}_{n-1} } {n^{2n-2}}$$ Using asymptotics $$\log(P_n)=\frac{1}{2}-\frac{2}{3 n}+\frac{1}{3 n^2}+O\left(\frac{1}{n^3}\right)$$ Continuing with Taylor $$P_n=e^{\log(P_n)}=\sqrt e\left(1-\frac{2}{3 n}+\frac{5}{9 n^2}\right)+O\left(\frac{1}{n^3}\right)$$ which shows the limit and how it is approached.
It also allows a shortcut evaluation of the partial products even for small values of $n$ as shown below $$\left( \begin{array}{cccc} n & P_n & P_n \approx & \text{approximation} \\ 3 & \frac{110}{81} & 1.35802 & 1.38411 \\ 4 & \frac{2907}{2048} & 1.41943 & 1.43118 \\ 5 & \frac{570024}{390625} & 1.45926 & 1.46553 \\ 6 & \frac{1873495}{1259712} & 1.48724 & 1.49097 \\ 7 & \frac{20872566000}{13841287201} & 1.50799 & 1.51039 \\ 8 & \frac{418915572075}{274877906944} & 1.52401 & 1.52564 \\ 9 & \frac{316401132826240}{205891132094649} & 1.53674 & 1.53790 \end{array} \right)$$
$\newcommand{\bbx}[1]{\,\bbox[15px,border:1px groove navy]{\displaystyle{#1}}\,} \newcommand{\braces}[1]{\left\lbrace\,{#1}\,\right\rbrace} \newcommand{\bracks}[1]{\left\lbrack\,{#1}\,\right\rbrack} \newcommand{\dd}{\mathrm{d}} \newcommand{\ds}[1]{\displaystyle{#1}} \newcommand{\expo}[1]{\,\mathrm{e}^{#1}\,} \newcommand{\ic}{\mathrm{i}} \newcommand{\mc}[1]{\mathcal{#1}} \newcommand{\mrm}[1]{\mathrm{#1}} \newcommand{\pars}[1]{\left(\,{#1}\,\right)} \newcommand{\partiald}[3][]{\frac{\partial^{#1} #2}{\partial #3^{#1}}} \newcommand{\root}[2][]{\,\sqrt[#1]{\,{#2}\,}\,} \newcommand{\totald}[3][]{\frac{\mathrm{d}^{#1} #2}{\mathrm{d} #3^{#1}}} \newcommand{\verts}[1]{\left\vert\,{#1}\,\right\vert}$ \begin{align} \prod_{k = 1}^{n - 1}\pars{1 + {k \over n^{2}}} & = {1 \over n^{2n - 2}}\prod_{k = 1}^{n - 1}\pars{k + n^{2}} = {1 \over n^{2n - 2}}\pars{1 + n^{2}}^{\overline{n - 1}} = {1 \over n^{2n - 2}} \,{\Gamma\pars{1 + n^{2} + n - 1} \over \Gamma\pars{1 + n^{2}}} \\[5mm] & \stackrel{\mrm{as}\ n\ \to\ \infty}{\sim}\,\,\, {1 \over n^{2n - 2}} \,{\root{2\pi}\pars{n + n^{2}}^{1/2 + n + n^{2}}\expo{-n - n^{2}} \over \root{2\pi}\pars{1 + n^{2}}^{3/2 + n^{2}}\expo{-1 - n^{2}}} \\[5mm] & = {1 \over n^{2n - 2}} \,{n^{1 + 2n + 2n^{2}}\pars{1 + 1/n}^{1/2 + n + n^{2}} \over n^{3 + 2n^{2}}\pars{1 + 1/n^{2}}^{3/2+ n^{2}}}\,\expo{-n + 1} \\[5mm] & \stackrel{\mrm{as}\ n\ \to\ \infty}{\sim}\,\,\, \,{\pars{1 + 1/n}^{n} \over \pars{1 + 1/n^{2}}^{n^{2}}}\,\bracks{\pars{1 + {1 \over n}}^{n^{2}}\expo{-n + 1}} \\[5mm] & \stackrel{\mrm{as}\ n\ \to\ \infty}{\Large \to}\,\,\, \lim_{n \to \infty}\exp\pars{n^{2}\ln\pars{1 + {1 \over n}} - n + 1} = \expo{\color{red}{1/2}} = \bbx{\root{\expo{}}} \approx 1.6487 \end{align}
Note that $\ds{n^{2}\ln\pars{1 + {1 \over n}} - n + 1 \,\,\,\stackrel{\mrm{as}\ n\ \to\ \infty}{=}\,\,\, \color{red}{1 \over 2} + {1 \over 3n} + \mrm{O}\pars{1 \over n^{2}}}$.