Impossible to answer without a DETAILED description of ALL markings.
No, they are readily available.
Yes you can. I have shot some in my dads Rem 7400. If you do shoot it. treat it likes it is corrosive and clean the gun promptly and properly.
100-400 USD
500 USD or so.
Yes, The receiver pins are the same as the Remington 870, so any stock that will fit the 870 will fit the 7400
Remington's website states manufacture started in 1981. Call Remington.
300 to 500 depending on condition and accessories. The rifle can be replaced for under $500
$500
DO NOT TRY THIS!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! You will cause great bodily harm and damage if you attempt to do this.
250-450
100-400 or so