Having heard the mono's of these two songs many times over the years, through bootlegs and other sorts of compilation, I was accustomed to these mixes. In the box set booklet, it's said that the Beatles mixed each album in mono, then did not partake in the stereo mix sessions. There was a gap of time in between mixing mono and stereo, so the engineers may have forgotten a particular specification regarding a track, or they accessed the tape of a different take.
The speed of the mono mix of SLH is definitely the way The Beatles intended the song to be heard. As Brendan pointed out, it was likely the engineer's (probably Emerick) decison to slow it down or it could have just been a case of them not comparing the stereo mix (which was done later) to make sure it conformed with the mono speed.
I'm not so sure about 'Don't Pass Me By'. At that point they were paying equal attention to the mono and stereo mixes so I wouldn't say the mono is definitive. Both versions are actually sped up from Ringo's original vocal but the mono is sped up a little bit more which may have been a mistake. I prefer the mono White Album overall but I just can't take that particular mix either.
I've always thought that Paul's voice didn't sound right on "She's Leaving Home" and had to have been slowed down. It definitely sounds natural on the mono mix. But I too am not so sure about "Don't Pass Me By".