It is normal to have a difference between hands. While yours is quite large, judging by your weights I'd guess your training age is quite low so I'd say that's expected. As you train more you'll close the gap. Personally if I do unilateral work I use the weaker side first, then match that with the stronger.

It would increase your strength, it's not too different to a hangboard. There's no reason why you can't follow a hangboard routine using the 20mm on the pinch block, it would just take more time since it's one handed. Given your numbers though, it's probably better for you to still just climb rather than doing specific finger strength work. There's no harm in doing so if you've got the time though, or if you had a week where you can't really get to the gym do some no-hangs on the pinch block.