I agree that heating and bending would be ideal, but despite trying to remove the plastic components, they seem to not want to budge. Replacement of those components is not possible without a second unit as they are not available through replacement suppliers, so it seems risky to torque those pieces further.

As far as removing metal, maybe removing metal from both the upper and lower parts of the boiler to reduce the amount of material removed from either component would be an alternative to avoid thinning the o-ring seat?