I can't seem to find anything clarifying this. The first purchase was a lease, (sort of) the second is generally described as a purchase. However, Boeing and expressed that it will entertain many options and there may be a buy back clause for countries interested in SH. But Australia may see fit to use them up putting 4th squadron off until 2030.
Link
Australian Government documents describe it as a purchase:
Link
I would assume by ~2025 the F-35 would make one heck of a ISR platform, its been put forward for years that the F-35 shouldn't be thought of just as a Fighter/strike platform, but as a ISR platform as well and work was being done to integrate sensor information in the ISR space.
Link
Link
I would imagine by 2025 a F-35 would be approaching or exceeding levels of EW/ISR that a growler could offer as well as being 1 type and cheaper to operate/upgrade than F-18 SH. Particularly with future developments after 2025 which growler wouldn't be receiving.
Your points bring up an interesting list of questions and possibilities.
With the original plan to only operate the Super Hornets for 10 years (then return them) having gone out the window for now, and as it appears they will be retained in service for at least 20 years, the question is, is there still any value in sending them back to the US when they are that age? Or do they have to go back regardless of age anyway?
The other point you mentioned, and have suggested, is that around 2025 the F-35 will be approaching or exceeding the levels of EW/ISR capabilities of the Growler without any future development of the Growler, but would that also be true once the Next Generation Jammer (NGJ) pods enter service on the Growlers in USN? I've also read that the NGJ pods will be able to be bolted onto the F-35's without any modifications, maybe that is the direction things will head in the US.
As to what happens here, assuming the Government (or future Governments) stick to the what appears to be the current plan, there's probably a couple of different paths that could be followed.
One may be that all three types (F-35, Shornet and Growler), operate side by side at least up until the Shornets are replace by the last Sqn of F-35's at around 2030, the open question would be if there was seen to be value in continuing operating the Growlers well past that point?
If the Growlers do stay in service maybe they will be upgraded with the NGJ pods and maybe it might be worth keeping the 'pre-wired' Shornets at least as attrition replacements, source of spares, buddy tanking role and in some sort of training role, at least for the 'basic' pilot training role of the Growler fleet.
Another path could see, after Shornets are replaced and the original planned 'all' F-35 fleet is in service, we could see the retirement of the Growlers shortly after that, and that capability passed onto the F-35's, would that be with or without NGJ pods being carried by the F-35's too?
I know those are all assumptions on my part, we are still a number of years away from seeing the first of the Growlers in service and even more years away from seeing the F-35's in service too, the crystal ball starts to get a big foggy at that point!!