The fact that you got different results between the system with the "+Y" versus the "Y" is not surprising. A "+Y" is a nonlinear support, it has no stiffness in the "up" direction, while it is rigid in the "down" direction. A "Y" on the other hand is rigid in both the "up" and "down" directions. As soon as you have non-linear supports in your system, the software must iterate for a solution. This does not occur if you system consists of only linear supports.
During static analysis, which one is more valid, using support type Y or +Y ?
Neither, they both have their applications. You should use which ever one is or will be actually in the real system in the field.
How about the using of Gap for stopper or guide ?
Again, these have their applications. You should use what actually reflects the real system.
is there any theory regarding this matter ?
Only experience and company procedures / policies.