if the MCLR is tied to VDD, how come my pulldown + switch are working as it is.
Because when you configure RA3 to act as a normal input the MCLR signal is disconnected from the pin and instead connected to Vdd internally. In other words, it's not the PIN that's connected to Vdd but the MCLR-signal. Does that make sense?

if I write 0, word B, C, does it mean the address 0 and 1 will contain the word variable B and at address 2 will contain variable C (byte)
Yes.

/Henrik.