Is (C) engraved on a piece an indication that it is Cobi brick?
The symbol stamped on the parts you are asking about is the international copyright symbol &copy; and is common on nearly anything with a copyright and not indicative of any particular brand or producer. A decent description of the use and purpose of the symbol can be found at http://www.copyrightlaws.com/international/522/

The tops of Cobi bricks have "Cobi" stamped on them. They also have a limited range of non-brick/plate parts available and have not duplicated many of the Technic parts, including the part in your image (at the time that I am writing anyway).

LEGO produces many parts in many different kinds of plastic and so while a part might not have the same feel as a standard 2x4 brick it doesn't mean that it isn't a LEGO part. The Bionicle/Hero Factory series of sets in particular use a lot of parts molded in plastic that is much less stiff than the ABS used for the System bricks. You were very close in your identification of the part in your own answer as "Technic, Axle and Pin Connector 3 x 3 with Axle" (Though the correct BrickLink part number is 111b, not 113b), however the actual part is instead "Technic, Pin Connector 3 x 3 with Axle" since they appear to be sloped on the ends rather than being a standard width for the entire part. The official LEGO name for the part is "32175 FOOT WITH CROSSAXLE" (Found in LDD v4.3, Brick Version 1182).