Thank you iFixIt. Your tutorial gave me confidence to move forward and get inside my computer. I agree with bhodges2, a great opportunity to clean. I'd keep a can of compressed air handy next time. I would also use a separate compartment for each screw. I treated the case like a wheel, lightly tightening each first. Still, the 3mm screws do not seem not entirely interchangeable
. Mixing them around, not forcing anything, eventually worked out.