JTAG boundary scan registers
Ad 1) In most cases yes, but I wouldn't say it is always like this

Ad 2) Typically, a Pin will consist of multiple boundary cell register entries (think of one register controlling input/output and another register entry controlling output value - both for a single pin). This strongly depends on the type of the cell. There may also be linkage/internal pins where you do not get a corresponding boundary register at all or pins where you are only able to read (sometimes this can be seen at oscillator cells).

Ad 3) This is all given in the BSDL file. A tool like XJTAG or TopJTAG will read the BSDL file and display the contents in an easily readable format.

Regarding the statement in some other post that "sometimes BSDL files are not up to date" I have to strongly disagree. A BSDL file is usually an output of the chip design, not something you do later in the process. Within the last years, I haven't stumbled upon a single BSDL file which was "wrong". Some of them were for pre-series production parts, others maybe got the boundary scan cells redefined in some later version but none of them really had serious flaws.