Are you sure about the bolded? Looking at various electoral laws, it seems pretty murky. Neither the Alberta
Election Act nor the
Election Finances and Contributions Disclosures Act says what happens in the event of two parties merging. If the Alberta Chief Electoral Officer uses his discretion and rules the way you're suggesting, I could see the new party fighting it by using other jurisdictions' electoral laws. The
Canada Elections Act says that merging parties need to notify the Chief Electoral Officer, and satisfy him or her that "the merging parties have discharged their obligations under this Act, including their obligations to report on their financial transactions and their election expenses and to maintain valid and up-to-date information concerning their registration." The s.240(7) of the
Saskatchewan Election Act -- which seems like it'd be the most relevant legislation, since it was in place when the Saskatchewan Party formed (the current Canada Elections Act wasn't in place for the PC-Reform merger, and besides that the PC Party was broke when Reform took it over) -- seems to allow federal political parties to make donations to provincial parties, provided they give the relevant personal details.
Are there any other relevant statutes out there? This is all me just making assumptions, but I feel like it would backfire on the NDP/Elections Alberta if they tried block the merger from happening on those grounds. Maybe the new party would be forced to offer refunds on donations, but I have a hard time imagining it going further than that.