Some other possible reasons for launching with the V6:
1) Lotus have a long standing relationship with Toyota and probably have a better margin on that engine than the AMG so they’ll want to kick of production with the more profitable option.
2) As others have said, minimise unknowns in the launch car, they can be confident not only that they can reliably mate that engine but also servicing is established so no additional work to do there immediately.
3) From a media perspective the launch reviews will be of the higher powered car with the better sounding engine and (likely) a manual. Those first reviews stick and that’s the review they want people to see.
4) By launching with the V6 they mitigate all the moaning about lack of character from existing Lotus owners!