No Lava in the Pressure Cooked Lava Cake!

There is a complicated formula for calculating cooking time based on energy input, surface area, thermal conductivity of food etc. Smaller pieces with a larger surface area will cook slightly quicker than large single pieces

In general for roasts (large slabs) just add the weight of the pieces and cook for the minimum time recommend by your pressure cooker for that (or from your experience). Then test using an accurate thermometer and cook some more as required. In my experience most people overcook meat anyway, so try it a little more undercooked for a change. Remember to let it stand at least 5 minutes per 500 g too

There are too many variables to give a blanket answer (stove energy, pressure in pressure cooker, meat type, fat content etc.)

What you need for the conversion of collagen is a certain amount of energy. It is a complicated process - the melting point is around 70°C for the type of collagen contained in beef, but the melting does not happen instantly once the meat reaches 70°C. In a pressure cooking, you can apply the same amount of energy in a shorter amount of time. This is not bad, as opposed to slow roasting of collagen-poor meat.

In collagen-poor meat, you have two types of protein, which are soft and wet. Under heat, they curdle, becoming tough and dry. The perfect meat is when the first type has curdled (so the meat is not raw) but the second hasn't, so it still holds juices inside. If you curdle both, your meat gets tough and you can't take it apart with your teeth.

In collagen-rich meat, you curdle both proteins - the collagen itself is tough and you want to melt it, but this happens long after the meat has curdled. But because the muscle fibers are not clinging to each other, but separated by collagen, you still get tasty meat. For that, you melt the collagen into gelatin, and serve the meat warm, so that the dry fibers are separated by the smooth, juicy melted gelatin. Unlike slow-roasted meat, you don't have to tear the juiceless fibers apart, and the gelatin makes up for the missing meat juices which were expelled from the cells during curdling.

So, in slow-roasted meat you don't want to cross the temperature limit for curdling a certain protein, this is why you have to apply heat slowly until the center of the meat has cooked, without the outside getting overcooked. In collagen-rich meat, there is no upper limit at which the meat gets non-tasty, so you can push the energy needed for the collagen-to-gelatin conversion quickly into your meat. The pressure cooker can do this better than the normal boiling process.