I'm wondering if it's possible to get from Krabi to Kanchanaburi without going to Bangkok.
Failing which, assuming I catch a bus for Krabi-Surat Thani or Trang, to catch a train an overnight train to Bangkok— what is the easiest and quickest way to get from Hualamphong to that inconveniently located Southern Bus Terminal?
Easy question first - yes there are direct buses from Bkk to Sangkhlaburi, though they do pass through Kanchanaburi. Buses leave Bkk from the northern bus station (Morchit).
Second question, Krabi to Kanchanaburi, perhaps by some kind of direct tourist minibus thing, but govt buses I highly doubt it. Instead would suggest bus from Krabi to Surat then train north.
Quickest way from Hualamphong to Sai Tai Mai is taxi - figure on around 100B depending on traffic.