Here's a complete proof of:

> Every subgroup of $\mathrm{GL}_n(\mathbf{R})$ containing $\mathrm{SO}_n(\mathbf{R})$ is either contained in the group of similarities $\mathbf{R}^*\mathrm{O}_n(\mathbf{R})$, or contains $\mathrm{SL}_n(\mathbf{R})$.

(This is equivalent to the statement that for every $t\in\mathrm{GL}_n(\mathbf{R})\smallsetminus \mathbf{R}^*\mathrm{O}(n)$, the subgroup generated by $\mathrm{SO}_n(\mathbf{R})\cup\{t\}$ contains $\mathrm{SL}_n(\mathbf{R})$. The answers to Q2 for $n\ge 2$ and to Q1 for $n\ge 3$ immediately follow.)

Let $H$ be a subgroup of $\mathrm{GL}_n(\mathbf{R})$. Let $\mathfrak{h}$ be the set of $x\in\mathfrak{gl}_n(\mathbf{R})$ such that there exists a $C^1$ map $u:\mathbf{R}\to\mathrm{GL}_n(\mathbf{R})$ such that $u(0)=I_n$ (the identity matrix), $u'(0)=x$, and $u$ takes values in $H$. Then $\mathfrak{h}$ is a linear subspace. Indeed, it's clearly stable under scalar multiplication, and if $u,v$ are two such maps with $u'(0)=x$, $v'(0)=y$, then defining $w(t)=u(t)v(t)$ we have $w'(0)=x+y$. 

Now assume that $H$ contains $\mathrm{SO}_n(\mathbf{R})$. 

First assume that $H$ is contained in $\mathrm{SL}_n(\mathbf{R})$; it follows that $\mathfrak{h}$ is contained in $\mathfrak{sl}_n(\mathbf{R})$.

Then $\mathfrak{h}$ is invariant under the action by conjugation of $\mathrm{SO}_n(\mathbf{R})$. Now (see the post by Venkataramana), we can decompose $\mathfrak{sl}_n(\mathbf{R})$ as a $\mathrm{SO}_n(\mathbf{R})$-representation, namely $\mathfrak{gl}_n(\mathbf{R}) = \mathfrak{so}_n(\mathbf{R})\oplus\mathfrak{p}$, where $\mathfrak{p}$ is the set of symmetric matrices with trace zero. Then for all $n \ge 2$ (including $n=2$), $\mathfrak{p}$ is irreducible; it follows that the subspaces of $\mathfrak{gl}_n(\mathbf{R})$ containing $\mathfrak{so}_n(\mathbf{R})$ and invariant under conjugation by $\mathrm{SO}_n(\mathbf{R})$ are $\mathfrak{so}_n(\mathbf{R})$ and $\mathfrak{so}_n(\mathbf{R})\oplus\mathfrak{p}=\mathfrak{sl}_n(\mathbf{R})$.

If $H$ is contained in $\mathrm{SO}_n(\mathbf{R})$ then we are in one of the first cases. Conversely, if $H$ is not contained in $\mathrm{SO}_n(\mathbf{R})$ then since the latter is the stabilizer of $\mathfrak{so}_n(\mathbf{R})$ in $\mathrm{SL}_n(\mathbf{R})$, we deduce that $\mathfrak{h}$ contains a conjugate of $\mathfrak{so}_n(\mathbf{R})$ distinct from $\mathfrak{so}_n$. Hence we are in the second case, that is, $\mathfrak{h}=\mathfrak{sl}_n(\mathbf{R})$. In this case we can pick a basis $(e_1,\dots,e_m)$ of $\mathfrak{sl}_n(\mathbf{R})$ ($m=n^2-1$) and functions $u_1,\dots,u_m:\mathbf{R}\to H$ as in the definition of $\mathfrak{h}$ with $u'_i(0)=e_i$. It follow that the differential of the function $(u_1,\dots,u_m):\mathbf{R}^m\to H\subset\mathrm{SL}_n(\mathbf{R})$ at zero is surjective. Hence its image contains a neighborhood of $I_n$ in $\mathrm{SL}_n(\mathbf{R})$. Thus $H$ is open in $\mathrm{SL}_n(\mathbf{R})$ and hence by connectedness of the latter, is equal to $\mathrm{SL}_n(\mathbf{R})$. 

Now remove the assumption that $H$ is in $\mathrm{SL}_n(\mathbf{R})$. If $H\cap\mathrm{SL}_n(\mathbf{R})=\mathrm{SO}_n(\mathbf{R})$, then $H$ is contained in the normalizer $\mathbf{R}^*\mathrm{O}_n(\mathbf{R})$ of $\mathrm{SO}_n(\mathbf{R})$. Otherwise, we deduce from the previous case that $H\cap\mathrm{SL}_n(\mathbf{R})$ equals $\mathrm{SL}_n(\mathbf{R})$, that is to say, $H$ contains $\mathrm{SL}_n(\mathbf{R})$. This finishes the proof.