They are cracking because they have been painted in. Frame and panel construction works by allowing wood movement in the panel, if you glue the panel down (with paint in this case) the panel can't expand and contract with seasonal changes in humidity and then cracks instead. Unless you free the panel it will continue to crack with seasonal movement. If you want to replace them probably the easiest way is to cut away the part of the frame holding the panel in, then replacing it with a molding when replacing the panel. I'd do that by making a template and using a pattern routing bit to cut away the old wood. That said, it would be a lot of work to fix a minor defect. An alternative would be to free the panel so it can move again (thin paint scraper around the edges), glue the panel back together and use something like bondo to smooth it. Disassembling an old door is difficult. You can cut it apart and then reassemble using loose tenons, making up the lost width of the saw kerf somehow, but you'd really have to love the door to make that effort worthwhile. If it were my door I'd ignore it, it's part of old house charm and a reminder to always accommodate wood movement.

Most of those cracks at the wall are clearly some places where it was caulked previously and the caulk has pulled away. It those cases you should use a box cutter or a sharp utility knife to cut away the old caulk first. Otherwise, parts of it will look like crap because you will need to apply more caulk than necessary, and the old stuff will possibly keep pulling away over time. If you haven't caulked before, here are a couple of tips. Use white latex painter's caulk with a small bead. When you caulk, do a few feet and then immediately go over it with a moistened paper towel stretched over your index finger. That will remove excess, push caulk into the crack, and smooth everything out beautifully. And latex caulk is great because you have a couple of minutes to remove it with wet toweling if you really make a mess of it, so you can start fresh. Here's a video showing how it's done; notice that the guy with the caulk gun does not try to apply a perfect bead since the wipe down takes care of any unevenness: How to caulk As for the cracks in the ceiling, that's an entirely different problem. It looks like someone put in a drywall patch and didn't bother to use drywall tape to bridge the seams. Thse cracks are the inevitable result. Caulking that will not be a good long term solution. You need to reapply joint compound and drywall tape to those, then sand and repaint.

It is probably from the house settling at some point, and might not indicate anything that needs attention (especially since it has been stable for some time), although I usually see such cracks emanating from a corner of the window rather than straight across the wall. Because the crack divides the wall, I would be on the side of caution and get a pro in there to look for issues in and around that wall. Things like slab subsidence or termites come to mind. Also, check outside to be sure that all water is directed away from the house - water collecting at the foundation can cause shifting in walls, and that does need to be addressed.

On the door facing outwards? Match either the glass bathroom knob directly opposite or the other knobs in the house. Reproductions or vintage salvage (architectural salvage, eBay, Etsy, antique shops....) Because your doors are not in a vacuum; match up with what exists in the rest of the house & ignore the modern interpretations of what goes with Shaker. Unless, of course, you just want to change all of the other existing hdwe in the house.