I think the SBR rules are fine, except if you play with techs and get jet fighters, the defending interceptors should roll 2 or less ( not one) if defending for SBR.
Also, sorry to go back to last section but…
I think i got a better idea on diplomacy phase.
Use Turkey for example… pay 6 IPC and need a 6 to score pro-allegiance. A player should not have to pay more money if they fail, rather you get one roll each turn like tech.
HOWEVER, you also have the option of paying another 6 IPC ( Turkey as example) to OBTAIN a lower threshold on die roll ( 5-6). At this point you paid a total of 12 IPC and roll once each turn 5-6. You can even pay another increment of 6 and get to 4-6 for success.
Also, what you pay is only for that neutral, you cant switch to another neutral ( unless you pay that money too)…the money you pay is only for that neutral you originally selected.