Hi Samantha,
Nice overview, I posted earlier a similar: https://williamvanstrien.blogspot.com/2020/05/how-to-include-external-as-presenter-in.html
A difference is that of the first step to add the presenter to a Teams in your tenant. This is not per se required, but what is required is that the presenter to join the Teams Live Event (via the internal meeting) must have switched in Teams to the organizing tenant. And the step 1 suggested by you is an approach that enables to switch in Teams App to organizing tenant.
One alternative approach is to explicit invite the external within the started internal “production” meeting. This is actually also (and the only) approach to include external presenters in Yammer Live Event: https://williamvanstrien.blogspot.com/2020/05/how-to-include-external-as-presenter-in_31.html
For both Teams and Yammer Live Events holds that including an external presenter is only possible with Teams production. With “external app” it is current not possible, due this is produced via Microsoft Stream which in the current “classic” edition does not support guest access. Once “new” Stream delivered and all Live Events consolidated on same platform this limitation is expected to be resolved.