In question 1:
By “tied notes” do you mean “let ring” notation? There is an option to apply “Laissez vibrer” ties in the Note and Rest section of the Properties Panel once you select a note to apply it to.
For question 2:
Grace notes (slashed or unslashed) are available.
For the symbols, you can start by looking in SMuFL
Note these are available in Dorico in various places, depending on how you are trying to implement something, and can always be copied and pasted into a regular text item — just change Character Style (first box in text popover menu) to Music Text before pasting.
I need tied notes, like laissez vibrer to notate brush swishes
But I need to have these ties defined as a playback technique, so they can trigger different sample.
When I want to add a slur with this dialog box, and select “Beams and slurs”