How can I stop a porch crack from causing internal cracking?
You have to jack the porch up to get it back to the correct height.  I would suggest using a level to hold at a congruent part of your roofline or front molding.

Take a car jack and a 4x4 and prop the roofline until it is a bit higher (1/8") than level.  Slide under some pressure treated wood or rubber shims under the posts.  Check in local rafters, connecting pieces and make sure that everything is snug, braces are still work, and so on.  You need a really good visual inspection.  This is a bit of an emergency since if you wait you could incur a lot of damage.

Now for the porch... There are a TON of ways to go about this.  You could just leave it alone and hope it doesn't sink anymore.  They won't sink forever even with bad footings.  There comes a point where they pretty much stay.  Just make sure the dirt around it isn't eroding - if it is landscape to protect the area.

You could also have a new porch poured.  They would simply jack up your posts, demo old porch, and pour new.

You could build a wooden porch over old.  This would probably require a little demo for the posts.  This is only feasible if your porch is a bit lower than your step - or you have to do a total demo.  Demo isn't as hard as you think because you can dig a hole and bury most of it in most cases.

People do have their pads injected with concrete to provide makeshift footings.  Concrete company will come out, drill through porch in strategic areas, bore deep enough for footing, and pour.  The good companies that do this have it down to a science and do great long lasting work.  However most never give you a proper footing and it sinks again in 5-10 years.

Really the take away is get it jacked up and then figure it out after.  This happens all the time and isn't the end of the world unless you let it go.