True, but even if you need 5-10 heroes, it's not much better than 56 diamonds each. Maybe 50 diamonds or something. I'm too lazy to work out the odds right now, but the math is on my site if you want to do it yourself (under Gold Chests).
Soul Mart is a nice consolation prize, but what you get there isn't as good as soulstones. It's also very hard to value Soul Mart purchases without doing some elaborate math that I don't want to bother with. But wait... You use your soul tokens to buy decanters? Why? Do you try to max the level of every hero or something? I'm not sure I've ever run out of decanters, in 3 years of playing.
You can. The Super Token Chest pays out at 26.7 diamonds per 100 tokens, and the Jackpot Chest pays at 40 diamonds per 100 tokens. If you consider Soul Tokens to be as good as any other type, then you can get better payouts with the All Hero chest (3.33 diamonds per 100 tokens) or random, low-level Elite farming with Double Drops (1.5 diamonds per 100 tokens).
Despite my disagreement with several of your finer points, I don't disagree here. Soul stone chests are very useful to players who have a lot of unfinished heroes.
I just did the math for the Boss Battle chest if you're interested -- http://zioth.com/dragonsoul/chests . It's pays out at 284 diamonds per stone!