Thank for all the background.
There are two ways to "transfer" or "upload" the video. It reads like you are using iPhoto and iMovie to do that importing and manipulation. More on this later... Good for you for backing up your data. I'll try and start with the last set of questions.
1) I suggest recording in highest quality. To use your words: "want the best possible video...why not...its available". Also, one can alway downsample and reduce quality - but not the other way.
2) USB is fine - The HDMI terminal is out-bound only (for connecting to an external monitor) not inbound. If you are interested in using HDMI to bring video into the Mac, check into the external boxes from Black Magic Designs. I *think* they would connect to a Mac using Thunderbolt - I don't think your Mac has a Thunderbolt port, but in case it does or you decide to get a new Mac, it is worth knowing about. USB2 is fine it is what is on the camcorder and the Mac.
3) You said your options for importing to iMovie are stabilization; Full (original size) or Large (significantly reduces file size with little image quality loss). The way iMovie works with stabilization is that it "crops" the video so there is an area that can be used when stabilization is needed and the edges of the screen are not black. It is a great feature and works well - but you lose some video. If the image was stable (like it a tripod was used, so no shake), then you just lose video. Personally, I would import the video with no change and Full. The "Full" setting maintains the best available quality. The lack of anti-shake maintains the integrity of the video and you can always include anti-shake when editing. When the video is imported to iMovie, it is prepared for editing, so it is decompressed. That's why the size balloons so much. We can go into how the compression happens - and how the decompression makes the files so large, but we can save that for later.
4) Stabilization is important - if it is needed. Use of some sort of a steadying device (tripod, shoulder-mount, chair, rock, table, etc provides stabilization so the software stabilization is not needed. You might not be a "pro" but you can capture video like them. If you have seen news crews around town or the camera crews at sports venues, 99% of the time, the *good* video comes form a camcorder on a tripod, shoulder mounted or on some sort of stabilizer - not software stabilized after the fact.
5) There are lots or tutorials at apple.com and on YouTube. Just start watching them and experiment. I like Final Cut because of the multi-track video and audio capabilities. If you think you want to do something in particular, look for that specific capability. We don't know where you are. There may be a Macintosh User Group in your area - they can be lots of help, too.
6) You listed what we needed. We can extrapolate processor by the age. Ballpark is fine.
7) The SDHC chip you use in the camcorder? Apparently - you keep filling them. If it was not the right chip, the camcorder would tell you.
8) Your Time Capsule back up is great. Whether it is "best" depends on your needs and budget. The only "better" method would be to use a Network Attached Storage system with multiple huge drives in a RAID1 configuration. This way, if the computer fails or is stolen, the NAS has the data - and if one of the drives dies, the other has the same data and you hot-swap the dead drive for a working one and it replicates itself from the working one...