sounds like you are fixated on spacers being the root cause of your problem as described. if the rim just needs to be adjusted left or right by tightening or loosening spokes (aka dishing) as has been suggested here by multiple posters, the mechanic you took it too is right. spacers aren't the issue. spacers can be judged to be appropriate with NO tire, rim or spokes on the hub. so there is no connection between the use of spacers and the problem you describe. spacers serve to position the hub between the dropouts such that when the cassette is mounted there is sufficient chain clearance between the small cog and the dropout and a match to the rear spacing.