How can we improve WHMCS?

Hey,

I have some questions related to domains registrars like Godaddy, Enom, namecheap, ...

1.
Is it possible to customize the forms(css) I got via WHMCS, the forms fields can be changed ?

2.
When there are methods that are not found in the WHMCS's Enom API( or with any other company), is it possible to add my custom method to my own server and combine it with the integrated API i got from WHMCS?

3.
When any event accord via WHMCS platform, can I get a callback of that changes? for example, if a new domain have been created/updated by my website's users, can I get a callback with the data?
Or the data is saved into the provider it self like Enom, Namecheap,.... ?

4.
If any upgrade happened to WHMCS plugin, will this force me upgrade the plugin?

5.
When my user try to buy a domain in my website, how the payment process will happen? because I need to put my own payment methods outside WHMCS?

Thanks a lot

p.p1 {margin: 0.0px 0.0px 0.0px 0.0px; font: 12.0px 'Helvetica Neue'; color: #454545}
p.p2 {margin: 0.0px 0.0px 0.0px 0.0px; font: 12.0px 'Helvetica Neue'; color: #454545; min-height: 14.0px}