What is the best method to write this?
I got this far by setting 2/3 and using hidden triplets;
Is this the correct/best method?
Can I get ( ) around 2/4 ?
Yes, I think this is your best bet.
For brackets, this might help:
If you have a project with lots of staves, don’t forget to make it system-attached text.
In fact, if you use @dan_kreider 's suggestion about the MusicGlyphs font (which is fantastic), you could just create a 2:3 tuplet in the “2/4” bars.
It’s easier to leave the outer staves in 6/8, so you don’t need triplets. Just put a local 2/4,3 on the middle staff, and then on the next bar a local 6/8 on the same staff and hide it. Then you only need hidden duplets (or 4:6s) on the 2/4 staff.
Won’t the barlines for the 2/4 bar not align with the 6/8?
When you write 2/4,3 that means show 2/4 but with a 3-quarter pickup bar. Then you silently put it back in 6/8 in the following bar, so all the barlines still line up.
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Gotcha
It seems to not work this way for the local time signature? Unless I’m doing something wrong
Edit: Don’t worry - I’m a fool. 
The 2/4 is just for show, of course. Since it needs parentheses, it’s probably just as well to do it as text and space it manually, like you said above.
I tried this but got bar numbers in [ ] in all the following systems.
Edit: It is correct now. Don’t know what happened.