Allow my rather anecdotal answer, as for the third point in the question.
I used to ride a stock Trek VRX 400, did XC riding along with some jumps and drops. I was 18-22 years and I think my weight was around 120 lb. At that age the bike felt perfect, I loved that bike, I felt the bike very responsive, and each time I wanted to learn some new move, I felt no "resistance" from the bike.
Some time later, My VRX broke, so I acquired a Diamondback X2 (this is a 1999 model, I bought the frame in 2007, I think...). For some reason I also inherited another VRX (Trek VRX 200). So now I had two bikes. I used to ride both in the same trials and trying the same stunts. The X2 had very lower end components compared to the VRX yet I liked it much much better, I'll tell you why...
By that time I was 24, (I'm 28 at the time of writing) and my average weight had raised to 160 Lb. (Today it's 175). I had a little more strength by then, I used to try to measure it pressing a scale against a wall with my legs...
Now, the bike comparison:
The VRX is noticeably lighter than the X2.
The VRX is a lot more flexible. With just sitting on the bike, and applying a litle force to the right pedal I could see the hole bottom bracket part go to the left, It was evident, I mean, you could literally see the frame twisting.
Both bikes had the same crankset sizes, but the VRX had a 11-32 cassette, the one in the X2 was 11-34.
The two bikes had some components that where the same, as I used salvaged parts from my broken VRX to assemble the X2. Had the same shifters, same rear derailleur, pedals, and tires. However, the X2 had V-brakes, and lower end hubs and rims. The inherited VRX came with disk brakes, bontrager rims, a bontrager front hub and a DT Swiss rear hub. Both bikes where ridden with WTB Velociraptor front tires @40psi. The VRX had a Velociraptor Rear (specific) tire at 40psi. The X2 started with a generic tire and was later upgraded to a WTB Prowler XT. both @40 psi. The X2 was initially assembled with the Manitou XVert-t2 fork from the broken VRX400 and was later updated with a RockShox Tora 302, While the VRX200 was rode with a Manitou Xvert DC.
While riding, the X2 had far better acceleration and climbing. This was not because of the cassette, because I shifted to higher gears in the X2. For example, a hill I climbed using 2-2 (2nd front gear, 2nd rear) in the VRX, has easy for me in 2-4 or even 2-5 with the X2. With the X2 I was even faster than a friend who rides a Trek Y 26 superlite (Carbon), the same guy beated me up if I rode the VRX.
I the descents I quickly became more agile with the X2, mainly because of two aspects. The perceived stiffness of the X2 frame made me feel the bike had a faster response to my input, I also got a much better feel of the ground through the bike. For example, with the X2 I was able to tell exactly when a tire was about to skid.
I became so confident using the X2, that I started Downhill Racing with it, something I felt very reluctant to do with the VRX.
After about a year of riding both bikes, the VRX had turned into my backup bike, used only when the X2 was on the repair bench or once in a while to avoid rust...
To summarize, the Stiffness in my X2 made feel much more confidence, and provided me way better "communication" to and from the bike, much faster response and terrain feedback. I could tell exactly where the tires were passing. This developed into much more maneuverability, despite the X2 having a slightly longer wheelbase.
The VRX, although had a light feel, is simply inefficient, I was never able to ride the same slopes with the same times. It made me feel tired more quickly, and had a dorky sensation along rocky ascents.
In conclusion: the stiffer frame outperformed the other enough to compensate for being heavier, and as a whole bike, it also compensated the lower quality components.