Don't worry about buying more replacment clips -- if you still have them on the left edge, not having the clips on the top and bottom isn't noticeable.
When reassembling at the last step (inserting the screen assembly into the back cover), do NOT simply do the reverse of what you did to take it out. Instead, insert the right edge first (with the plastic tabs). The metal tabs on the left edge (and top and bottom edges if you have any left) are flexible, so it is very easy to snap the left edge of the screen into the back cover AFTER the right edge is in place. If you put the left edge in first, you'll have a much tougher time trying to get the plastic tabs on the right edge into back cover.
I see that the author broke 4 tabs on the top and 2 tabs on the bottom -- the exact same ones that I broke. I must be following his instructions perfectly!
It would be really helpful to point out where all the clips and tabs are, and to show a picture of them (unbroken), so you know what you're trying to release.
I used this guide to open up the printer and replace the network interface card. Just do everything up to Step 4, then
* unscrew the two screws on the back that secure the parallel printer port
* unplug all cables (take some pictures first, so you remember where each cable goes)
* unscrew the four (or in my case, five) screws that secure the card in place
* the card can then be carefully worked free
Suggestions for Step 3:
* Use a plastic spudger to work the top of the side cover free
* Take care with the plastic tab on the lower right (front of machine) so that you don't break it when pulling off the cover