Disc 4 loose ends - hope you can help!

Disc 4 loose ends - hope you can help!

I've just completed Disc 4 and I already have most of the side quests but I still haven't got the Skill Master achievement for any of the immortals or the Treasure Trove (I'm on 900 points).
I know part of the problem is I'm missing 4 seeds. I started the game thinking they were some sort of booby prize and didn't bother much about them, then I assumed the missing ones were the funny seeds you pick up occasionally and finally I assumed that you could probably buy them in the auction in Saman after you leave an area. Wrong, wrong and wrong!
I'm just going to go back over everywhere forensically for the seeds I reckon but I'm wasting a lot of time on two other things and I'm out of ideas.

1 There is still an unactivated platform in the Crimson Forest (the large one just before the Sorcerer's Shrine) and I just can't work it (I make sure Mack is in the party when I go looking)
2 I can see two unopened chests in Numara which I can't get at. One is across the canal when you go down the alley at the side of Ninn's Boutique (it's not the one you get to by going up the ladder, across the roof and down another ladder - it's across the water). The other you can see down the side of Nalia's, across a canal again. How do I open up that part of town?

Appreciate your help!