The kind of "Why don't you...?" question in (2) is not an information question, it's an offer (as it is in this case), a gentle suggestion/request, or an invitation. It's not like the real information question "Why don't you have a cell phone? "
Look at these examples, noting the contrast between "some" and "any":
Why don't you have any potato chips? You were going to buy some. Did you forget to put them on the shopping list? (information question)
Why don't you have some potato chips? These are a new kind--baked instead of fried
"How come" is only an information question. It is a shortened version of "How does it come about that [(X) is the case]?" That's why the word order is statement order, not question order. "How come" introduces a real information question, and is similar to "Why...?" The speaker is asking for an explanation.
The differences can be seen in these examples:
I'm going to be too busy today to order tickets for the concert. Why don't you do it for me this time? (request for action)
(Later) How come you didn't order tickets? You knew I was going to be too busy today. ("Why didn't you...?")