It is possible for a Server licensed user to author a viz in a browser without using Desktop at all. However, there are currently some limitations (think the primary Desktop workspace without any righ-click or popup screens -- e.g. no editing/creation of calculated fields, limited formatting, no editing axes, adding trend lines / forecasting, etc...) And it is only for editing a viz and not editing/creating a dashboard.
Eventually, I would suspect that all or at least most Desktop functionality will be brought into web authoring via Server. For now, it is doubtful that anyone would be able to accomplish what they wanted from an authoring perspective using Tableau Server only.
A version or two from now that may be different. Will that cannibalize Desktop licenses? Maybe. But I suspect the trade-off of selling more Server licenses and also making Tableau authoring more accessible to every platform (iPad, Unix, anything with a web browser) is fairly desirable.
Your understanding of licenses seems correct (but as always you may want to check with your sales rep )
Thanks a lot Josh.