We use 要 when we ask for sth or we need sth such as 'I want to have... 我想要...' and ' I need... 我需要...' . When we use 有 that means I have or own it. For example, 我想要你嘅電話號碼 means I want to have your phone number while 我有你嘅電話號碼 means I have your phone number.

I am sorry that I don't understand your second question. Do you mean putting the verb *after* 可唔可以？

For me, 我要可唔可以 (putting the verb before 可唔可以） is grammatically incorrect, so the proper way should be 我可唔可以要. But, it will be ok if u say sth like 我要你嘅電話，可唔可以啊？that means 'I want to have your phone number. It's that ok for you? '.

I think the 要 means 'have' in this sentence. It's verb in this sentence so we couldn't speak without this word. The proper way to say 'Can I have your phone no.?' is '我可唔可以要你嘅電話？'or '你可唔可以比你嘅電話我？' or '我想要你嘅電話，可唔可以啊？'. We put the verb 要 after 可唔可以. I hope this will help.