Regarding the 4th point: I plan to do this seriously. For example, you have problems with electricity. Or you need to leave. Or you need PC resources for another application.
The smoothness of the UI is a very important thing and it's good that you noticed it.
There will be Task Manager, created specifically for the GUI. You can close the program, and after opening it will restore the analysis and all running processes. You can pause the analysis / testing, etc. The analysis process can be saved either completely (with the process status, hash), or partially (saving the PV and after the engine hash table is correctly filled with many quick launches).
Now I am developing an interface design. This is a big job and I have a lot of ideas that will have to go through a public test

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The idea on the engines: first, auto-update. For open source this download and automatic compilation. Then - for each engine - its version history. Also, for each version - the ability to store different settings of the engine. This idea can be further developed, but now it is not the main thing.
Next - the ability to view the game in real time with all output (this is for the game engines). I think this is a more specialized thing.
Also, the best work with PGN. You can simply bring the move to the PGN (and this can be a PV engine, or a move from the analysis of the game) and open it in a separate window for analysis.
Resource management: you can have many positions to analyze. In this case, you can do the testing engine, as well as do some kind of tournament with the broadcast to the Internet. Task manager himself will say that you can run, and that - you can not. You yourself allocate available resources.
Regarding testing engines: here you can make it possible to test on different PCs, but combine one result.