I haven't painted in oil for a long time, so I will only answer those questions that I remember the answer to. Let's start with the chalk primer: two layers are sufficient, but the application must be accurate. You have to apply it with a wide, soft brush, if you apply the first coat with vertical strokes, the second coat you have to apply with horizontal strokes. You have to apply it with long, even strokes (the movement starts from the shoulder), apply the first coat, let it dry, apply the second coat, let it dry, then sand the surface with fine sandpaper and wipe off the dust with a cloth. Yes, the brushes you bought are very cheap, so it's normal for them to lose their bristles or hold their shape badly, and the bristles are probably very hard (which is one of the reasons why the underlying layers of paint come off). For cleaning, rinse in white spirit first, then with soap and water. Once clean, shape the tip with your fingers so as to put the bristles back in place as much as possible, dab lightly with a cloth or kitchen paper and dry them by putting them in a glass or jar with the tip upwards: this way they should last longer but I still suggest you invest in some better quality brushes. I prefer soft bristle brushes but it really depends on the result you want to achieve. Now, drying times:  for the underpainting I suggest you use a colour very diluted with turpentine, as if it were a watercolour, this way you will have a background colour that will dry quite quickly, the same for the sketch. Today there are new generation mediums that have different drying times. I'll link you to a video showing some of them. https://www.youtube.com/watch?v=lZf9QaTxDfM&list=PLdE0Y1KGg4JoyeiaOUAy458D5ryrxKb7z When I was at school we only used linseed oil, which takes a really long time: the more you added, the longer it took for the colour to dry. The reasons why the colour comes off when you try to apply new colour can be any number of things: you've used too much medium and literally the colour slips off it, you haven't let it dry enough, the bristles on the brush are too hard. Oil painting, however, is a very time-consuming technique, and oil painters often have more than one work in progress at the same time, so they can switch between them while the colour dries. I used to prefer to apply it with a palette knife, without medium, but it's not a technique I've ever been very fond of, which is why I've always preferred to paint in gauche. Oh, and another thing occurred to me: as I understand it, you also thin the colour with a solvent in the subsequent underpainting stages, which dissolves the underlying layers! That's why you use medium to thin oil paints. The point is that it doesn't make much sense to paint in oil and then use it as a watercolour, it should keep its mellowness. If you want the colour to dry faster, don't use the medium and apply a small amount at a time, almost using the dry brush technique. If you want a technique where the colour dries quickly, I suggest you switch to gauche or acrylic.

I'm not telling you to not use medium, thinners and whatnot, but I personally only use my oils straight out of the tube. Adding flat color to a large area thinly but opaquely (ie. shadow or light of a sphere) and then adding steps to soften the transition afterwards works for me and I can get quite far without paint slipping or not being able to push it around enough (also I find that you can work a small-ish area from start to finish in one sitting). If I cant seem to blend two colors I would add more paint (in a color between the two i'm trying to blend) and then try again. I find it easier to get softer transitions with softer synthetic brushes but natural hard bristles will get you far. My hog bristle brushes lost a few hairs in the beginning as well, but it's wearing off.

I'll try to provide some pointers: 1,2. Oil paint doesn't dry, in the sense that oil evaporates; oil oxidizes, i.e. it reacts with oxygen, which is a slow never-ending chemical reaction. The more oil you add, the slower it oxidizes, the more thinner you add, the faster. If you stick to the 'fat over lean' rule, the lower layers will dry relatively fast. But, you are right, layering oils takes time. 3. A medium changes the properties of the paint. That's all. There are mediums for shortening the drying time, prolonging it, which makes sense for acrylics, glazing mediums, and so on. So, yes, you could call thinners and oils mediums. 4. For starters, oil paint doesn't feel like paint, so it takes a while to get used to it. 5. When cleaning brushes, make sure you get rid of all oil and thinner, and use a conditioner to get your brushes into shape; there is special brush soap for this, as hand soap tends to dry out your brushes like nothing else. Good brushes don't cost a fortune, simple hog hair brushes will do the job, unless you go for a real smooth finish. 6. Yes, gesso works that simple.

Jan, welcome to a long, long road of learning. I paint in acrylics so I'll hit the answers that I know and leave the rest to others. Your brushes are horrible and will lead to insanity. It may be the way you are handling them, but they were very cheap, and that may be the problem. Some brushes come with a bit of gum arabic on their bristles to protect them in transit and maintain their shape. Just stand them briefly in warm water and gently work it out. That said, water is a HUGE enemy of oil brushes, and will swell the fibers and make them unruly. Synthetic brushes don't do that, because they don't absorb water and are the choice of acrylic painters (who use a LOT of water!). Oil painters found synthetics sensitive to the cleaners that were used, such as turpentine and mineral spirits or gasoline (not a typo!!), so stayed with natural fibers, such as hogs' hair. Paint that builds up at the base of the bristles also causes the splaying that tips the ends out, as will cleaning them and leaving them to dry with the bristles pointing up (water seeps down into the ferrule) . Dry flat or hanging tips down. My go-to for brush cleaning is Windsor and Newton's brush cleaner, which is nothing short of a miracle compound, and safe. I have also heard of methods (for oils only!!) where you don't clean your brushes, but rather dip them in something (ask Google!) that keeps them from drying out for a few days and avoids cleaning altogether. I would suggest that you go to an art store or two where they will allow you to handle and test drive the brushes. Personally I would go for a synthetic. There are newer ones that have the body and spring you need for oils and can cope with water. You also need to find out which brushes you like to work with: long or short bristles, flat or round, curved ends (filbert, my fave) or blunt/square. It is a long process, but start with a couple of medium sized, decent quality brushes and find out what works for you. The other problem is your substrate: it is probably way too smooth to hold paint. Get cheap practice pads and use both sides of the paper until it starts to make sense for you. Then you can find ways to make your own surfaces. That said, your study has a lot of really good stuff happening! You are well under way. Lay your paint on a textured, gessoed surface with a good brush (that doesn't scrape off paint with the ratty bristles) and you are going to find it a rather blissful experience!

I only have very little experience with gesso though. Apparently yes you can put that on almost any surfaces. Because the nowadays the binder inside that is mostly acrylic/plastic, they should adhere pretty well unless on some special surfaces like maybe some metal or kitchen wraps. Cesar Santos (on youtube) do that to his drawing books I believe.