Regarding 1 & 2 - I suggest always keeping the API docs handy - https://clear.ml/docs/latest/docs/references/api/definitions

I love using the API since it's so convenient. So to get to business -
To select all experiments from a certain project you can use tasks.get_all with filtering according to the API docs (I suggest you also use the web UI as reference - if you hit F12 you can see all the API calls and their responses. This can really help to get an understanding of it's capabilities I'm not sure the runtime sits in the database as an exact number, but you can easily calculate it as 'completed' minus 'started' time of experiments.