1. How strong do you think South Kaioshin was? Why did Kid Buu absorb him?
We KNOW that Buu absorbed South Kaioshin after just killing West and North. We also know that South Kaioshin was the strongest of the Kaioshin, being 'somehow' absorbed by Buu. But that's all the manga really tells us.
The Z sword sequence would seem to suggest he's weaker than SS/SS2 Gohan; Gohan is able to pull out the Z sword that "not only [Kibito] but numerous Kaioshin were unable to free". However, it's never stated or really even heavily implied that South Kaioshin tried. On the other hand it would be a bit odd if he didn't, if the sword really was as legendary as East Kaioshin made it out to be. But then, there's also the anti-Kaioshin seal theory, which has some basis as it was put there by a magic using enemy of the Kaioshin.
There's also the anime filler scene of Kid Buu and South Kaioshin's battle; South Kaioshin blocks and counters some of Kid Buu's hits and gets in a couple of shots, but is easily taken down when Kid Buu lands a solid kick on him. This one kick appears to do a ton of damage; South Kaioshin is covered in wounds afterwards and has trouble standing afterward. The way the anime has it, Buu appeared to be outclassing South Kaioshin handily but absorbed him because either he recognized his strength as worth taking or was just feeling lulzy.
The manga may imply something else with Kibitoshin's "somehow"'; he didn't say this about Dai Kaioshin, and already knew how Buu absorbed people, so it seems unlikely he's referring to Buu's method of absorbing people. It seems like he's considering South Kaioshin's defeat as something that "somehow" happened.
Additionally, there's Mr. Buu. He should contain both the Kaioshin that were contaminating Buu, so he could he possibly tell us South Kaioshin's strength? I go by him since it seems to coincide with the above anime sequence; I have South Kaioshin making up the vast majority of his strength, with Dai Kaioshin's just being a small addition. We saw that Mr. Buu was strong enough to put up a fight against Kid Buu (unlike Vegeta) in the vein of Goku vs Freeza or Super Buu vs Gotenks, but Kid Buu obviously outclassed him. Still, it didn't seem like a MONSTROUS difference.
2. How strong do you think Buff Buu was?
I've usually believed that Buff Buu was only a bit above Super Buu based on the following quote:
Chapter: 507 (DBZ 313), P13.5
Context: as evil Boo reverts to his South Kaioshin form
Goku: “H-hey…Vegeta…His ki is increasing, ain’t it…!?”
Goku has trouble telling that Buu's ki is even rising. This to me indicates it's not that big of a jump in power; maybe 10% or so. However I've seen some interesting points on Buff Buu brought up. Mainly, look at his head tentacle:

The length may appear longer than it is because Buu is bending his legs here, but Buu's head tentacle is quite long. Not Buuhan long, but still long. Some people propose that the head tentacle is proportionate to his strength, which isn't completely baseless. Compare Evil Buu, Super Buu, Piccolo-Buu, Gotenks-Buu, and Gohan-Buu.





This may suggest that Buff Buu rivals Ultimate Gohan; his head tentacle goes down to his waist or so.
3. Was South Kaioshin's absorption completely unique?
Stuff in favor for it:
1. Kid Buu didn't get South Kaioshin's shirt
2. Buu's body changed significantly, unlike with other absorptions.
3. Buu's facial features didn't change.
4. Kaioshin energy couldn't be used to resurrect Buu.
5. Super Buu was implied to be multifolds above Kid Buu through Gotenks.
Stuff against:
1. Buu's absorptions were explicitly established as additive earlier by Buu himself.
2. South Kaioshin's absorption was never stated to be different, and Toriyama is a simple author who doesn't like to leave things too cryptic.
3. The Daizenshuu implies the Gotenks stuff was false.
4. Power Level obsessed folks only: do any of you have a nice looking formula for this?
Here are my current levels on this whole thing:
SS3 Goku- 32,000,000,000
Kid Buu- 36,000,000,000
--South Kaioshin absorbed- 126,000,000,000
South Kaioshin- 27,000,000,000
Grand Kaioshin- 2,000,000,000
Mr. Buu- 29,000,000,000
Super Buu- 112,000,000,000 = Fat Buu (full power)
Ultimate Gohan- 200,000,000,000
SS3 Gotenks- 150,000,000,000
(yeah, based on their fight, I have Gotenks way above Super Buu)
I have the absorption of SK at (A + B) x 2. That looks kind of sloppy to me. I'm wondering if someone can come up with something more... "stylish", or "elegant", I guess, like TheMightyOzaru's two digits theory. If you can keep my exact levels while doing this, and not make Super Buu more than 5 times stronger than Kid Buu, and integrate that same system to Dai Kaioshin weakening Buff Buu to Fat Buu's full power (which I think is equal to Super Buu, especially considering what the Daizenshuu said), then all the better.