Your lawyer is absolutely right. Any delay by the vendor will result in the completion date to be extended in favour of you as the purchaser. The common understanding in SPAs is that:

Vendors want their money fast and conclude the deal AND Purchaser has the duty to make payment of the balance purchase price on/before the completion date. As such, any delay by purchaser and he has to pay late penalty interest to the vendor. Any delay by the vendor, and the purchaser gets more time. Vendor is at a loss as the market price of the property may go up. Factor in inflation as well and the depreciation of money - both factors whereof are detrimental to the vendor.

If upon completion of the SPA, the vendor fails to deliver vacant possession to you - then he has to pay late penalty interest to you. That is how it works.

So now, either you back out of the transaction or you be patient and agree to the clause to wait until April for redemption of the vendor's loan.

Usually, upon redemption, it takes 3-4 weeks to complete the SPA, so you're just looking at another few more weeks of delay.

If you execute the SPA in 2 weeks' time (bearing in mind the holidays and a few exchanges of drafts) - you're looking at the date of the SPA to be around the first week of March. End of April would be around 7-8 weeks from the time the SPA runs. If all goes well with redemption, you're looking at another 3-4 weeks to complete it. The standard completion date for SPA is 3 months. So, you'd still be within time. If there is a bit of delay, it'll be for another few weeks only.

Weigh the pros and cons first before deciding. How much do you want this property? Is the deal and price good that you won't be able to find another?
If you think you can't wait for it, then back out now. Good luck.