With Chromatic Orb jump ability can it be used with War Caster reaction?  I for one am think it can as it only targets one and the leap is a secondary effect that may or may not happen.

I would rule no, since the second hit also requires an attack roll. If I allowed it, I would void the opportunity for the second hit when cast as a reaction with War Caster.

I would say that you can cast Chromatic Orb with Reactive Spell since the spell does in fact target one creature. The bouncing effect is a bonus effect that applies outside of War Caster's Reactive Spell use requirements.

A DM might not allow you to use this spell as a reaction, or do the bouncing bc of action economy and their combat balancing, but as per RAW, it seems totally doable. Also rule of cool, it's awesome to see it in action! If your caster player is constantly doing this strategy, punish them harder for being in melee range and no longer having their reaction available to them. They may also burn through their spells faster. There's plenty of ways to handle balancing against this without merely telling your players "No." Chromatic Orb only bounces if targets are within 30ft, just space out enemies. It's also a separate attack roll each bounce, enemies could utilize cover more to lessen bounces. And let's not forget that since the caster player is within 5ft of the enemy, the ranged spell attack would have Disadvantage, unless they also had the Spell Sniper feat.

The player is committing to a feat choice if they're doing this. While War Caster is a solid feat, there's much better feat options to take. Plus, your caster player isn't guaranteed to be using their Reaction on Reactive Spell each turn! They may have already used/have to consider using Shield/Healing Word/etc, and if your enemies have decent INT, they may catch onto the caster and simply not leave melee range so as to not trigger a Reactive Spell. And if all that wasn't enough, Counter Spell still exist.

So be creative DMs! Let your players have fun with this little synergy! It's not game breaking, and I really do believe RAW allows it. If you feel otherwise, that's fine. Discuss with your group and see what everyone thinks. Don't forget that it's YOUR game, you guys can play however you want at the end of the day. May your fails be few and your crits plentiful! Be safe adventurers!

Based on the way they ruled old Twinned Spell, no, you can't. Any spell that can potentially target more than one creature, even if it won't in the current circumstances, is ineligible, and the 2024 definition of target is a lot clearer than it was before.

I might allow it anyway, but Ice Knife would definitely be out

I'd say yes you can cast c.orb but it will not bounce to another creature.

Given that as per the rules of Opportunity Attacks the attack is made while the creature is still within melee range, your c.orb is made at disadvantage. Even if you allowed bouncing, it is still a poor choice. Just use True Strike or something.

To beat that melee range disadvantage when spell casting you need the spell sniper feat. Now why spell sniper has it and not war caster I don’t have any idea as warcaster would be a far better place for it. The only reason I can see is that Tasha’s is they both have 3 benefits beside the asi and if war caster had the casting in combat then it would have 4 and spell sniper 2. But you could give spell sniper something else like advantage on hiding after casting.