I'm absolutely not an authority in these matters, and any corrections are welcome and appreciated.)
From what I've read, "green" hydrogen made from electrolysis with energy from non-carbon-emitting sources extremely energy intensive, as is compressing it for transport. Transport is also expensive. To my knowledge, there aren't any pipelines in Western Canada that could transport it, so I guess it's trucks and train cars. But maybe there's potential there because hydrogen wouldn't need to be the backbone of the grid, it would just need to pick up the (substantial?) slack when renewables weren't producing.
Seasonal leveling doesn't require transport -- make the hydrogen above the storage caverns.
Green hydrogen certainly is more expensive than hydrogen derived from fossil fuels, but if we are imagining a 100% RE grid, that hydrogen isn't available. It does make sense to burn natural gas directly instead of burning green hydrogen as long as the CO2 charge isn't high enough to rule out use of natural gas. This is why we're not yet seeing a large green hydrogen role out in Europe.
The interesting question is whether fossil hydrogen with CO2 sequestration is acceptable. It may be preferable to green hydrogen, depending on things like methane leakage.
An alternative solution to hydrogen could be methane synthesis from captured CO2 and green hydrogen. The infrastructure to transport, store and use methane (aka 'natural gas') is already there.
This is possible, although the problem is the cost of capturing the CO2. It might be better to synthesize a liquid fuel; methanol is probably the easiest. Capturing the CO2 of combustion is probably easier than capturing atmospheric CO2, so that should be included. Overall, the RTE will be lower than hydrogen, but maybe it's cheaper if the geology doesn't favor hydrogen storage.
Another possible alternative would be artificial geothermal, with very high temperature rock at fairly shallow depth. Obviously that's not transportable.
