MCS-51 available internal ROM

"I was under impression that 8051 could be manufactured with a desired program in the internal ROM"

The ROM was part of the manufacturing process - so it had to be manufactured with the program in!

"If the internal ROM was used entirely by the chip itself and useless to a programmer, there would certainly be no need for ROM-less 8031 and 8032 microcontrollers, would there?"

I don't get your point there.
The point of the ROM-less versions was for the cases where you could not commit to manufacturing your code, so you just had to have it in an external device - usually UV-EPROM.

"Also, there's the EA control signal which would be quite useless if internal ROM was used only to store some sort of 'system code'"

Again, I don't get your point there.
EA allows you to use external Code memory instead of any internal ROM.

"The reason for my dilemma is that I read on this website that software reset is performed by jumping to code at C:0000h."

Address C:0000h is where the hardware starts execution after a hardware reset.
So, jumping to that location will cause the code to start from the same point as after a hardware reset, but will not actually do a hardware reset.
Hence it's probably better to call it a re-start than a "reset"...

"I'm wondering if this is where the entry point of user's program is stored"

It is where everything must start - so, in that sense, it is the "entry point"

For a 'C' program, it would go immediately to the startup code - which ends up calling the user's main()

http://www.keil.com/support/man/docs/c51/c51_ap_startup.htm

"I was under impression that 8051 could be manufactured with a desired program in the internal ROM"

The ROM was part of the manufacturing process - so it had to be manufactured with the program in!

"If the internal ROM was used entirely by the chip itself and useless to a programmer, there would certainly be no need for ROM-less 8031 and 8032 microcontrollers, would there?"

I don't get your point there.
The point of the ROM-less versions was for the cases where you could not commit to manufacturing your code, so you just had to have it in an external device - usually UV-EPROM.

"Also, there's the EA control signal which would be quite useless if internal ROM was used only to store some sort of 'system code'"

Again, I don't get your point there.
EA allows you to use external Code memory instead of any internal ROM.

"The reason for my dilemma is that I read on this website that software reset is performed by jumping to code at C:0000h."

Address C:0000h is where the hardware starts execution after a hardware reset.
So, jumping to that location will cause the code to start from the same point as after a hardware reset, but will not actually do a hardware reset.
Hence it's probably better to call it a re-start than a "reset"...

"I'm wondering if this is where the entry point of user's program is stored"

It is where everything must start - so, in that sense, it is the "entry point"

For a 'C' program, it would go immediately to the startup code - which ends up calling the user's main()

http://www.keil.com/support/man/docs/c51/c51_ap_startup.htm