The EA-18 is hardly obsolete, in fact the US navy was still taking delivery of them just over a year ago.
The EA-18 isn't obsolete but the ALQ-99 is.
They also ordered more F/A-18s despite the F-35C being available.
To put that in context the fleet size of the SH is over double the expected fleet size of the F-35C and that was always the plan. The SH is also currently cheaper to acquire and with the issues the USN is having with fleet aircraft availability and airframe hours (a result of heavy tasking and a low investment in maintenance and depot capability) it makes perfect sense to acquire more SH in the short term. Despite those acquisitions, the SH is slated to leave the fleet by 2040. I expect come the middle 2030s the USN will continue to take F-35C, or whatever variant is available then, due to the same reasons it is taking SH today.
Additionally, the 'Next Generation Jammer' pod will fly first on the EA-18, with integration on the F-35 deferred indefinitely. (though it seems likely that the F-35B will get some added equipment, since the marines don't operate the F/A-18)
The USMC operate the F/A-18, just not the EA-18. The NGJ will almost certainly migrate to the F-35 but at the moment it is not a priority given the USN now has an adequate fleet of EA-18.
It is almost certain that SH Blk III will be an upgrade and not a new acquisition program. Once the USN and Kuwait take their last jets the line will, if Boeing don't win Finland or India (and Germany doesn't buy), roll over to upgrading existing jets to the Blk III standard.
Further, it seems counterintuitive to put a powerful jamming radar on a stealthy asset. All you do is illuminate yourself. Stealth seems much more useful for passive sensing and delivering your payload. Not to mention that bulky non-stealth pods under a stealth jet significantly increase its detectability.
That isn't a good understanding of how a stand-off jammer, which the EA-18 is, works. In the context of stand-off jamming the host platform being stealth or not isn't really important. It also depends on what type of jamming the platform is conducting, whether noise or deception. The issue remains though that either once the aircraft is detected or while it is moving around the battlespace having a stand-off jamming capability on a stealth platform would make that aircraft more survivable and probably allow it to position itself for the most effective jamming.
As for the external pod, the expectation is the NGJ fitted to the F-35 would likely reside within something like the Terma Multi mission pod, which would preserve stealth characteristics, with a small increase in overall RCS, over the current NGJ pod. An internal fit is possible but would have to engineer around the probable heat issues.
https://www.terma.com/press/news-2012/f ... n-display/
The F-35 Pod Enclosure will provide real estate on the F-35, which can be used to expand the F-35 Special Mission functionality, by allowing the F-35 to fly Next Generation EW and ISR systems, such as Jammers and EO sensors.
The thing to note with any SH acquisition by Germany is the fleet likely becomes unsustainable past 2040 when the USN retires its fleet. No amount of spares or tapped out airframes sitting in the desert will overcome the main sustainment issues of mission systems and avionics. Noting also that the SH is not a more upgradeable platform than the F-35 and it is likely less able to be modified by Germany than the F-35. Germany gains no greater access to source code with a SH over an F-35 while the F-35's systems and software are designed for third parties to integrate with.
Realistically, the plan ahead for Germany should just be more Eurofighters and give up nuclear sharing.