How can I secure / fasten a half installed floating engineered hardwood floor?
I've seen this opening of seams and it regularly happens with a "snap together"-type floor. Unless the manufacturer states the flooring can be secured it would be wise not to chance any repercussions that may occur. The working mechanics for a floating floor system is to allow the planks to expand and contract freely (that's the reason for the 3/8 inch spacing along the wall). If  no allowance is made for movement the boards will move in the plane of least resistance, usually upwards. I found that it's easiest to close the gap by kicking the board tight with a rubber soled sneaker. Nice looking floor BTW.