KVL with multiple source
1) Yes, you could also reverse the polarity of the voltage source and write +5V instead. It makes the schematic easier to read. Both variants are equivalent.

2) If the polarity of a voltage across a resistor is not known, it is required to make an assumption. A minus sign indicates that the actual polarity of the voltage is against the assumed direction. But this is not "wrong" because the result is just with respect to an assumed polarity.