Bold part: Covalent character means that the ions within an ionic compound are distorted, causing the ions to end up in a closer proximity to one another in the lattice than would be expected with the perfect ionic model. The fact this brings the ions closer means greater attraction between the ions, so more energy is needed to separate them, indicating greater bond strength.
Italic part: Not true. Covalent bonds tend to be stronger as there is overlap between the participating atoms, meaning the electrostatic attraction between the nuclei and participating electrons is very strong. Try adding water to most covalent compounds under ambient conditions and you can generally expect little to no bond breakage. Try adding water to your typical ionic compound and it should dissolve, indicating that the bonds have been broken (albeit only temporarily).
Underlined part: Polarity leads to partial positive and negative charges, so naturally you get stronger electrostatic attraction. This means a stronger bond overall. The more polar the bond, the stronger (in theory) it should be.