Schmitt trigger offset voltage
No it would not be the same if you remove R1 and use a voltage source to drive V+ pin. The V+ threshold voltage would be fixed and there is no way the output feedback resistor can change the threshold voltage.

To calculate the voltages, you just have two cases. When output is high, the feedback resistor is pulling high to positive supply voltage so it is in parallel with R1. The other case is when output is low, it will pull the node down to negative supply voltage. You can use Thevenin to find out that the R1 and R2 boil down to 2.5V source with 50k impedance.