I can elaborate some of your questions.
1. A diamond(stop sign) shape eliminates sharp 90 degree corners, which spreads the stress points to 8 corners rather than 4, or a round platform which has no corners so the stress is spread as evenly as possible.
2. Vertical(or down-hand) welding is the weakest of all the positions. It flows nice and may look nice, but it doesn't penetrate into the steel worth jack-squat. The only times where this is admissible is if you have a corner to corner joint with a hairline gap, that way it'll still fill in nice, and penetrate to the joint's full potential. Or on anything under 1/8" plate, the thin plate should heat up enough for proper penetration. If it's not turning blue on the other side of your work piece, you're not hot enough.
It's not a matter of weakened steel, it's a matter of poor penetration.
3. Any and all welds(excluding aluminum) should be stronger than the base metal. Tempering and annealing are necessary in high carbon and alloy metals to reconfigure the HAZ(Heat Affected Zone), and the crystal base structure. In low carbon/alloy steel(mild steel), it's really pointless to do this because the HAZ is usually under control(unless excessive heat/welds are excecuted). At the steel mill, they can actually "burn" steel, it gets too hot and forms perminant, brittle, useless grain structures. I've read where they had to throw away 280 tons of burnt steel, no good.
4. The weld should be the thickness of the thinner piece being joined(EX. 1/8" to 1/4" plate, an 1/8" bead is all you need). What happens when you overheat steel with weld is the weld metal and the base metal form huge crystals that are inline with each other(they should be out of sync), and that makes it brittle and finally, failure. Had an example done at the college, took a stick welder, used 10 sticks to weld a 4" joint, and cut it in half. You could see the actual grains of the metal separating from each other.
On a truck frame, I wouldn't worry about it. I trust you have good judgement on what's strong and what's not.
5. Vertical up is one of the hardest welds to do. As you weld, gravity is constantly trying to pull the weld puddle down, which if done incorrectly, will look like the aftermath of an expired TV dinner.
It takes the right amount of heat, wire speed, travel, epecially gun angle, and travel speed. A slight weave is acceptable, but not too much, it'll cause unwanted stresses in the weld. The uphill travel heats up the metal as you go, making the weld penetrate better. This is what's used in the field for repairs on heavy objects, when moving or turning it is impossible, so multiple welding positions are required, and down-hand is a big no-no.
It's easiest on flux-cored wire, because the flux creates an instant "mold" that holds the liquid metal in place as it cools, forming a flawless bead. Solid MIG wire is a little more erratic, takes a lot of practice to get that one figured out.
Whoo, that was a lot of typing. I'm going to bed.