Ask your Member of Parliament to intervene on your behalf with HRSDC to get the S.I.N. issue straightened out. When an MP intervenes it becomes a "ministerial inquiry" and government departments are required to at least come up with an answer or explanation within 48 hours.Every federal government office has someone who's job it is to handle "ministerials". Whether you voted for your MP or not, or whether you like them or not...go to them. Resolving constituency problems is a major part of any MP's job. Its what you pay them for. Given that we're in the middle of an election campaign, I would think that your MP would have a bit of "extra" motivation to get your problem resolved.
After you get your S.I.N. problem straightened out, go back to your employer and try to resolve the situation. If they still won't pay up, file a complaint with the Employment Standards Branch of the Ontario Ministry of Labour against both of your former employers.
IANAL (I am not a lawyer) but I would think it would be a good idea to speak to a local community legal clinic about your situation. Community legal clinics often deal with EI appeals, WSIB appeals and Employment Standards issues.