To add to what Radu already said, much of which I agree...
1. As Radu mentioned, DOF will vary, but focal length makes a big difference, so yes, I'd still try to get the focus on the right centerpoint...as you use longer lenses like 300mm, the distance between objects in the frame can be quite large and still fall outside the DOF even at a smaller aperture.
2. Agree with Radu - No. Manual focus doesn't use any of the focus systems of the camera, so it doesn't matter if you've got 1 focus point or 100 cross points.
3. AF is generally pretty accurate on today's cameras - but the mode you are in (center, spot, multipoint), and the lens itself (some lenses backfocus or front focus) can make a difference. And the conditions and subject also play a part - focus systems can struggle with uniform color areas, lack of contrast or definition, or low light situations. It's good to use MF to be sure or to fine tune (many cameras or lenses allow you to autofocus, then rotate the manual focus ring to 'fine tune' the focus).
4. As Radu said, clean the sensor only when it needs to be cleaned. If you ever want to check how dirty your sensor is, find a nice section of fairly clear sky, focus to infinity, set aperture to a small number like F22, and snap a shot. That will usually show you anything that might be on the sensor (clean the lens first, so you don't confuse the two!). Start with a bulb blower, as it will usually work 85% of the time...then if you still need cleaning, try a brush or wet swab.
5. Nothing wrong with 3rd party lenses - some are good, some not so good, price usually tells you what you're getting. Quality control shouldn't be any more of an issue than it is with your brand lenses...everyone can make a lemon. Sigma, Tamron, and Tokina are all well established and reputable lens makers.
6. Not always...generally, the idea is that those lenses are better built and often with better coatings - so they are better than the cheaper versions...but they aren't always the equivalent of a camera manufacturer's high end glass. Remember too that not even all L glass is alike - some are better than others...same goes for the 'high end' lenses from the 3rd party manufacturers. Also, many times the L designation also brings weatherproofing, all-metal build, special cases, etc...the higher end lenses from the 3rd party manufacturers may have better build and optics, but usually aren't as heavily built and solid as the very high end manufacturer glass.
7. How nice the background is on a lens, the 'bokeh', is dependent on many things, not just aperture. The shape of the aperture blades certainly makes a difference, how fast the max aperture is can shallow the depth of field more quickly yielding background blur at closer distances, focal distance also contributes as does the distance between subject and background, and even just the optical design - sometimes background blur happens at the right spot, but just looks 'ugly'...other lenses produce a lovely, rounded, smooth appearance that prompted the 'bokeh' legend.
8. As Radu said...cheap is usually cheap. You might find a real gem - a very cheap filter that is actually quite good, but most of the time, you get what you pay for. I tend to stay safe, and spend a little more for the quality.
9. In general, raising the ISO will usually result in more noise than long exposures at lower ISOs. Obviously some cameras are much better than others, but if the ISO is low, an exposure can run for dozens of seconds with no real appreciable noise from the heating of the sensor, whereas increasing ISO will usually increase noise exponentially with each step up...or if using your camera's high ISO noise reduction may not show more noise, but lose details as the camera's noise reduction smears it all away. For night shots, most will use a slower shutter speed and low ISO if possible, as it gives the cleanest, nicest look with no noise. Night shooters only raise the ISO if they have to - for example, not having a tripod available, or shooting a moving subject that requires a faster shutter speed.
10. I use the viewfinder probably 85% of the time. As Radu mentioned, primary reasons are due to extra stability in my stance, easier to focus on subject in very bright light, easier to track a moving subject's motion, easier to maintain panning on a subject during continuous shooting, ability to see subjects in low light situations where the live view can't pick up enough, and battery conservation. However, I do enjoy using live view for shooting at odd angles, for tripod mounted shooting at night, or for waist-level candid style shooting. I can't say I would use live view with any other camera but Sony though - most live view systems are super slow, focus slow and poorly, and have huge delays between pressing the shutter and getting the shot. Sony has the only live view DSLR that can be used with no slowdown in focus or shooting speed...and that's really the only reason I use it. If I had another brand of camera, I'd probably not use live view much if at all.
11. Generally, even cheap entry level cameras always use a metal tripod mount, and most tripod screws will insert at least two full turns into the camera - as long as one occasionally checks that the screw is tightly turned into the camera, I would have no worries about the camera on the tripod. I walk around with a 9lb lens on my camera, mounted to a tripod quick-release plate, and often walk with the tripod slung over my shoulder holding the legs, with the camera and lens mounted to the top plate. I keep the necks trap around my hand just in case, but I've traveled like that for years.
Sony DSLR-A580 / Sony 18-250mm / Minolta 50mm F1.7 / Sigma 30mm F1.4 / Tamron 10-24mm / Tamron 200-500mm / Tamron 90mm F2.8 macro / Minolta 300mm F4 APO
Sony NEX5N / 18-55mm F3.5-5.6 / 55-210mm F4-6.3 / Pentax K adapter / Konica K/AR adapter / bunches o' Konica & Pentax lenses!