Where is the "Enable All Threads' Options in the "Left Navigation Bar' ?

@qqq Do you see "All Unreads" in the left navigation bar?

yes

when I click on it, I see threads I'm involved in in linear fashion

err, I mean I see an "All Threads", not an "All Unreads"

OK, so you already have "All Threads"?

Yes.

When I click on it, it shows threads I"m in ... in linear order.

Right, full width.

However, I was under the impression there was some config option where the threads are in linear oder, EMBEDDED in teh channel

No.

because "All Threads" only shows Threads I'm in. It doesn't show threads in the channel where I didn't comment on.

All Threads is about the message you've involved threading, it doesn't mean all the thread in the channels, hence this is expected behavior

Same is true with All Unreads