I am reading Purcell and Morin's Electricity and Magnetism 3rd Edition.
Equation ($1.22$): $$\vec{E}(x,y,z)=\dfrac{1}{4 \pi \epsilon_0} \int \dfrac{ρ\ (x^\prime, y^\prime, z^\prime)\ \hat{r}\ dx^\prime, dy^\prime, dz^\prime}{r^2}$$
On page 22, it says:
"A continuous charge distribution $ρ\ (x^\prime, y^\prime, z^\prime)$ that is nowhere infinite gives no trouble at all. Equation $(1.22)$ can be used to find the field at any point within the distribution. The integrand doesn’t blow up at $r = 0$ because the volume element in the numerator equals $r^2 \sin \phi\ d \phi\ d \theta\ dr$ in spherical coordinates, and the $r^2$ here cancels the $r^2$ in the denominator in Eq. $(1.22)$. That is to say, so long as $ρ$ remains finite, the field will remain finite everywhere, even in the interior or on the boundary of a charge distribution."
According to the above quoted paragraph, equation $(1.22)$ becomes:
$$\vec{E}(x,y,z)=\dfrac{1}{4 \pi \epsilon_0} \int ρ\ (x^\prime, y^\prime, z^\prime)\ \hat{r}\ \sin \phi\ d \phi\ d \theta\ dr$$
Here there is no particular direction for $\hat{r}$ at $r=0$. Then how can we say that in spherical coordinates the integral doesn't blow up at $r=0$.
I have more questions on this:
(2) How can we be sure that the integral doesn't blow up at $r=0$ in other coordinate systems?
(3) Are there any analogous expressions for electric field (independent of $r$) due to surface charge density and line charge density?