If all abutters are in favor of granting my variance/special permit application, will my application be approved?

No; ZBA decisions are not made based on the popular vote of everyone present.  The ZBA makes its decisions based on applicable state and town bylaws.  This may mean that something that is acceptable to all abutters that are present may be turned down.  Similarly, the ZBA may approve something that is unpopular with abutters because it is allowed by applicable law.