1) The current to be limited is the motor current; max 3A. The sense resistor voltage is not allowed to be higher than 2V, but at 3A this is 6W of wasted power.
And 0.5 ohms * 3A = 1.5V & 4.5W which is also very much. 0.22 ohms * 3A = 0.66V & 2W which starts to get acceptable. 0.15 ohms * 3A = 0.45V & 1.35W = ok.
I would choose to make each sense resistor out of two 0.22 ohm 0.5W resistors in parallel, and having a 0.33V reference voltage.
I would also make provisions for feeding the sense voltages into the L297 via 10k resistors, and having 220pF decoupling cap's at pins 13 & 14.
This is to prevent (any possible) noise from activating the current limit too early. The circuit layout is also critical to minimise ground noise.
2) That is called decoupling capacitors and is a universal, fundamental remedy to prevent short/fast voltage spikes from damaging IC's.
3) To quote the datasheet: "Ground noise problems in multiple configurations can be avoided by synchronising the chopper oscillators."
4) To quote the datasheet: "when the inputs of the IC are chopped ; Schottky diodes would be preferred."
5) To quote the datasheet: "not of a wire wound type". In other words: use ordinary 0.11 ohm 1W metal film resistors.
6) It looks ok as far as I can see, but I have not checked every little aspect nor built & tested it so I can't guarantee there are no problems.
7) Done.. Can I go to bed now?
No bother, it's ok, you seem to have done a lot of good homework.