RTM_CHANGE is supposed to operate on a single component of the multipath route (e.g. on a single nexthop). Search of this nexthop is peformed by iterating over each nexthop from multipath (nexthop) group, using check_info_match_nhop. The problem with the current code that it (incorrectly assumes that check_info_match_nhop() returns true value on match, while in reality it returns an error code on failure).
Fix this by properly comparing the result with 0.
Next, the followup code creating a new nexthop group, was modifying original group instead of a new one, leading to the spurious errors.