4. I am designing a product using P89C51RD2HBP (because it is easily available here).

I am exploring the possiblity, whether I can put the user
program code in external EPROM begining at (external Program) address 0x1000,
and my Bootup code in Internal Program Memory starting at address 0x0000

No, you can not.
The external/internal program bit (!EA) is sampled a power up only (read the datasheet) thus you can not switch between internal and external.

To do what you want, use the RB2 (16k internal) or RC2 (32k internal) since these chips switch to external program memory when the limit of the internal is reached.

Erik