Hello Farshid,
Regarding Question 1:It is not currently possible to create a string that only has a backslash character. I will let R&D know about this and post the CAR ID.
Regarding Questions 2&3: Unfortunately, there are no additional string manipulation TPL functions and definitely no filename manipulation routines. I'd suggest that if you must use pathnames, that you keep the path and the filename in separate variables and combine them only when you need to manipulate the file. That way, the TPL code and independently access the filename and path. Also, you might want to consider using user parameters, which is a way to get user data specified in a model diagram into TPL for processing. You could have a user parameter for the path and another for the filename.
I will file a product suggestion for more string manipulations and pathname functions.
Cheers,
NathanT