Group ID (as offsets change often because of varying sound lengthening, they're not used anymore) list is there mainly for those who are forced to use Brawlbox, but it can be used for quick referencing too. Why not.
Unless you're going to get the offsets yourself pretty much everytime, you'd better not.
The second group ID is not for offsets, it's for group IDs. If you're willing to use a hex packet to replace voice of for example Mario, you don't type the offset there, you type the group ID of snd_group_chara_mario - it's 8.
Now the offsets change everytime you're getting longer or shorter sounds in.
You don't use base wave, unless you continuously cannot get the sound to work and you've already tried saving the sound in Audacity.
If you can stuff that, I think you can, after all I got 40-seconds clip to work over announcer call.
Sound over-flowing happened to me only when the sound had a really weird frequency (it was exactly 13370). Try using more standard one and you should be fine.