Plan B, heh
1. Supporting the raid with a 4E attack out of Port Moresby is good. It will help to distract enemy CAP. However don't just limit yourself to flying on the 18th only. They should also go in on the 19th in support of the carriers.
2. Don't send in the TFs if they don't have full bunkers. Just in case the KB shows up, you will kick yourself if you can't high tail out of Dodge at flank speed.
3. Your plan to send in the carrier strike in the morning against the enemy airfield is OK if you know for certain that the KB will not appear. Otherwise you face risking a KB strik with no capacity of your own to retaliate. The safer course of action, one which incorporates a Plan B is to have your strike carrier aircraft tasked with "naval attack" as their primary mission and "airfield" attack only as a secondary mission. That way you;
(a) get a chance to sink any enemy ships loitoring in the vicinity of Rabaul, which in the long run will be of much more to you than destroying a few aircraft and damaging the runway which will quickly be repaired,
(b) minimise the chances that the KB will catch you with your pants down, aka your fighters will be less fatigued as a result of having escorted the bombers on an airfield attack in the morning,
(c) with the PM 4E having been given the airfield strike as their primary mission, they will go in first in the morning and help to fatigue/destroy the enemy CAP over Rabaul, thereby clearing the way for your afternoon carrier strike against Rabaul.
4. You plan to launch from the vicinity of Shortlands. It probably doesn't have an airfield present but that doesn't prevent you from flying in at the last moment a Kingfisher unit to provide some additional CAP over the carriers. All you would need is the presence of an AVD/AV or in their absence, some Aviation Support dropped off by Catalinas.
5. Shortlands would be too close to the enemy but you could preposition some tenders further back, say at Tulagi to give some triage to any cripples from a KB encounter. Even a nearby anchorage would suffice for triage.
6. Your idea of avoiding detection is good. Don't be afriad of aborting the mission if detected before launching. In that event you might want to move the carriers around to another location, albeit still under enemy surveillance, before retiring to safety. That way you help to "persuade" your opponent to engage in reinforcement rather than expansion operations in the near future, which is the raid's primary stated mission.