Well done, you've managed to salvage enough, but sometimes no amount of skillful hands and a bunch of knowledge can do anything, and some hardware ends up malfunctional in a recycling yard. That's how it is...

Indeed. Sometimes it's just not economical to repair something when the part is more than a replacement. Luckily I haven't had to replace any surface mount ram chips yet.

I gotta save up the stuff that needs careful and delicate probing for when I've got the patience and co-ordination. So I do the visual inspection first, and any pins look dull, fluxy, very lightly soldered, I'll clean with isopropyl then run a micro-torch (butane powered) round all the pins, (I guess I'd use a hot air pencil when I get around to buying one) and re-test in system. If it comes right it comes right, if it don't it's saved on the "when I feel like getting up close and personal" pile. Maybe about a 50:50 success rate with that, as first attempt to fix.

Using a micro torch instead of a soldering iron to reflow solder sounds terrifying. I wouldn't even attempt this kind of repair without the assistance of a microscope.

You don't hold it in one spot very long, just play it back and forth.

It's not just the flame but the fact that you can't see what you're doing as you do it. Even with good eyesight you would have to put your face close to where the flame is to see anything. Otherwise I think it would be more of a guess as to what the flame is touching.

The advantage of a microscope setup besides being able to see close up is to keep your face far enough from any toxic vapors so that a fume extractor can suck it all away.

Don't need to see, when it's cleaned and excess flux is burned off, solder will coalesce and wick into where it's supposed to be. Check after a couple of passes, try again, maybe throw a slice off a "solder splat" on there if there's not enough solder. Think of it as being like when my door hinges are squeaking I don't mess around with a stethoscope to determine which particular two surfaces need a drop of oil applied precisely with a toothpick, I just hit them all with the oil can.

edit: If you're wondering what the analog to the squeak is, it's that the pins "shine wrong" i.e. little or not at all, the pin makes a right angle at the pad, if there's no thin meniscus of solder joining the pin to the pad, that has been applied hot enough it looks too dull there, you might even notice the dark line. So the solder puts in a small fillet between the right angles of the pad and pin when applied sufficiently and is making good contact. No sharp right angle, looks different in the light.