BIQ-IQ questions

Aantat I would not say I am well versed in the BIG-IQ but I can definitely shed some light on some of your questions.

1Q. Do I have to Re-Discover and Re-Import configuration every time if I change config on BIG-IP devices.
1A. If you make changes on the BIG-IP itself you will have to re-import the configuration and make the BIG-IP as the configuration to trust when this sync occurs. It is easier to do a re-import and re-discover rather than just re-import.

2Q. What is the best practice for Making changes via BIG-IQ? Deploy it every time when I make changes (sounds stupid, I know)
2A. I'm unsure if a best practice exists but if you make changes on the BIG-IQ you should absolutely push those changes to the BIG-IP if you want them to be in place.

3Q. Can I do everything in BIG-IQ same as in BIG-IP? For the first it seems like I can't.
3A. You cannot. Some pieces or even entire sections of configuration from the BIG-IP cannot be configured under the BIG-IQ. An example that I know of is you cannot enable an F5 trunk through the BIG-IQ but you can set it up on the BIG-IP and then sync the BIG-IP configuration to the BIG-IQ.

As for the rest of your questions I would venture that they are all limitations of the BIG-IQ and would require making the change on the BIG-IP side and then syncing the configuration changes back to the BIG-IQ.