manicmike

Well, we're all about getting machines working well, and your questions have been pretty intelligent so far.

If you push in the plastic disc with the numbers on it, it releases the screw which will then come out. Lay the bits carefully in order (and/or take photos) as you go, and you should be able to clean the existing discs then put everything back as new.
Subsequent calibration of the tension is easy too. Set it to something, allowing the positioning pin to enter the plastic number disc (which holds them together). Set it to zero, thread the machine, put the presser foot down (to engage the tension). Adjust the hole the pin is entered into until you can pull the thread with barely any resistance (it has minimal pressure). It is now ready to go, so turn it between 3 and 4 and your thread should now be quite tight.
Lift the foot, put some fabric under it and test sew.

Just a couple of notes Rachael.
Firstly, since you put the locating finger (that metal bit) back yourself, you will need to make sure the gap is right. It should not be too tight or your thread will start breaking (a full rotation of the bobbin should not see it bind at all). If the gap is too large, your bobbin case will rattle too much (they will normally rattle a little). The gap should only be large enough to allow thick thread through.
Secondly, this class of machine sometimes sees the hook timing go out. You should familiarise yourself with how to set it to factory. They have also often had the hook timing deliberately changed by some idiot who thinks this is what you do to allow 15x1 needles to be used. Result of incorrect hook timing is always a very poor quality stitch.
Thirdly, The bobbin case can be modified to allow use of 15x1. My blog (see sig) has an article on how I did this, and I haven't has a bobbin case strike so far (in several months of using the machine almost daily).
Lastly, your name. My eldest daughter's middle name is Rachael, so ask any question you like in the knowledge that I'm totally biassed
Mike