Yep sorry that was a typo should be ZCB I will edit it.
To get BC correct you adjust C. Try adjust that in smaller amounts than Z, say 1cm increments. I have found loosening C can sometimes improve stability and power. Loosening C depending on the kite may destabilize the tips with collapsing etc. Depends if the bridle placement for C is more in the B area there. In such case tightening it would improve tip stability, but worsen depower control. Engaging C generates much more power than just B. Alternative is to adjust C at the tip. Often though tip instability is caused by Z shrink there, so extensions for Z at the tip are a better solution if extending it all doesn't give the desired result. Tightening C may improve L/D ratio so upwind, float etc, but you may lose power and some depower control or gain it if the tips are a problem.
Shortening A will make B looser than C. So you would need to lengthen C to correct it in ratio with B. You would also need to lengthen Z more to correct B and C to original levels. -2cm A shorter makes B +1.5cm longer and C +1cm longer. +5cm longer Z would on it's own make B -1.25cm and C -2.5cm shorter. Add them together and you have B +0.25 and C -1.5 cm. Like that things are a little out of ratio, overall you have reduced camber with C like that. To return to ratio you would need to lengthen C +2cm, this would quite change the kite giving it positive camber. To return the kite to the previous profile before adjusting A, you would need to lengthen Z +6cm total (so another +1cm) and C +2cm. In reality it is what ever makes you happy with the kite, but with A -2cm, C+2cm and Z +6cm you will have a kite with the exact same profile as before just more depower.
If you want to visually observe changes to the kite, you really need someone else to fly it. You can do it by yourself but it is difficult, requires experience. One thing to look for is when bridles go slack or tight. B slack indicates full depower on most kites.