Hello & wellcome, bondaree !
With CHDK you can set the Tv value either manually from a menu, with a shortkey (e.g. the zoom lever, camera dependent) or in a script.
The aperture can also be controlled from the menu or in a script.
Note: not all cameras has a real aperture, the Ixus (Powershot SD) series and also some of the low-cost A-series models have a ND filter instead; CHDK can control the ND filter on this cameras from the menu or in a script.
--> You can have a look to my wikia page
to see some (old) screenshots of the CHDK menu.
--> See also CHDK wikia: Lua script "Universal Tv mode"
as a sample for the powerful scripting possibilities in CHDK...
BTW A590 have firmware video setup of 640*480 20fps. As CHDK provides possibility of selecting video quality can it overwrite it to 30 fps
No, only the quality (compression) can be controlled, not the frame rate.