An editor ought to be able to determine whether a manuscript presents research that is within the scope of a particular journal, however this verdict is one on that journal staff tend to fall back when they don't have a better 'verdict'. Suppose, for instance, the staff have asked an editorial board member or two to assess a paper, but these academics have responded that the work is outside their expertise. The staff might then have chosen several potential reviewers to review the manuscript, but if no response has been forthcoming, the people handling the manuscript will need to decide what to do next. The manuscript can sit in a queue awaiting further requests to editorial board members who are increasingly less likely to respond that the paper is within their area of expertise, or the staff can return the paper to the author on the assumption that if the board members have declined to review the paper, the manuscript is probably outside the reviewing capabilities of the journal.

I would suggest providing a cover letter when resubmitting the paper elsewhere in which you (1) specify the general subject area for the journal staff, (2) give the name of the editorial board member who is best qualified to oversee the eventual review, and (3) choose different people as the proposed referees, possibly including a remark like "Any of the authors of reference 16 would be suitable referees for this work, as would Professor...", thereby giving the staff many names to choose from. Note that you should avoid suggesting close collaborators, or if they are indisputably the best qualified reviewers, explain why they are suitable as referees and how your relationship with them has changed if, for example, you suggest someone with whom you published in the last year or two.

The journal may have problem finding a referee that understands the paper, while it is an unpaid and tiresome work to function as a referee. However, if they come to the conclusion that the paper falls outside their scope you would expect them to reach this decision within some days.

An editor ought to be able to determine whether a manuscript presents research that is within the scope of a particular journal, however this verdict is one on that journal staff tend to fall back when they don't have a better 'verdict'. Suppose, for instance, the staff have asked an editorial board member or two to assess a paper, but these academics have responded that the work is outside their expertise. The staff might then have chosen several potential reviewers to review the manuscript, but if no response has been forthcoming, the people handling the manuscript will need to decide what to do next. The manuscript can sit in a queue awaiting further requests to editorial board members who are increasingly less likely to respond that the paper is within their area of expertise, or the staff can return the paper to the author on the assumption that if the board members have declined to review the paper, the manuscript is probably outside the reviewing capabilities of the journal.

I would suggest providing a cover letter when resubmitting the paper elsewhere in which you (1) specify the general subject area for the journal staff, (2) give the name of the editorial board member who is best qualified to oversee the eventual review, and (3) choose different people as the proposed referees, possibly including a remark like "Any of the authors of reference 16 would be suitable referees for this work, as would Professor...", thereby giving the staff many names to choose from. Note that you should avoid suggesting close collaborators, or if they are indisputably the best qualified reviewers, explain why they are suitable as referees and how your relationship with them has changed if, for example, you suggest someone with whom you published in the last year or two.

The handling editorial staff can have a decision within a week, maximum. But to keep it hidden for a long time without any decision is purely malicios and unprofessional

If the MS do not fits at all the aims and scope of the journal. I believe that it is unethical behavior to hold the MS for a longer time. The editor will be able to make a decision in few days.