Is Speed Baking 3 achievement possible without cheating

According to this thread at Xbox360Achievements.org, kills in the opening sequence do count towards the achievement.

Since my original link died, the IGN Strategize on Deus Ex: Human Revolution also states that kills during the opening (pre-augmentation) sequence do count against the Pacifist achievement.

The specific achievement you mention is obtainable as there are actually 10 character slots. You can also get this achievement by creating, leveling, then deleting characters (or selling them, after Blizzard allows characters to be sold on the AH).

Regarding the rest, it is indeed possible to get all achievements; none of them should be mutually exclusive.

In fact, here is a thread where a number of people discuss their attempts to get the "world first" for obtaining all achievements.

The exception is that, as @aolszowka mentions, some Feats of Strength are no longer obtainable. However, these are usually excluded from "all achievements," and in fact this is one of the reason Blizzard invented feats of strength.