It looks like you've made your decision to bed the blocks on concrete and this sounds sensible. Not only will it give your greenhouse the best chance of a long life (particularly if located in a windy spot) but it should also help you to get the blocks level which will pay dividends when it comes to fitting the greenhouse. I'd recommend using a 3 to 1 mix of sand to cement for this. Oh, and one more tip, try to get the corner to corner diagonals equal on your blockwork to ensure the squareness of the base. Best of luck.
The main problem with not having a footing is the risk of things like moles and rodents digging under the blocks. There's no real weight to a greenhouse though and it's a rigid structure so it's not likely to subside in any drastic way. I fit all mine on sleepers directly onto soil as they're long enough to bridge any holes.
If you can keep your head, while those around you are losing theirs, you may not have grasped the seriousness of the situation.
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