If there is nothing under the decking that makes it that height then lowering it is no problem.

The joists, the frame that the decking boards are fixed to ought to be of a size that will take the weight and weather forces that the summer house would bring to it. I always use 6" x 2" and then use enough of them for the weight the deck will take. Most decks have joists at anything up to 2' apart, so I would suggest no more that 14-16" apart to put a summer house on. If you are considering anything like a hot tub, then add more support to the frame by adding 'legs' that go under the 6" x 2", not nailed to the side of them.

So long as the timbers are OK, then reusing them is fine.

7 x 4m is a big deck that is raised but of course, as long as there is no interference underneath then technically it could be lowered. Some new timbers may need to be added to replace damaged or rotted ones but yes, it could be lowered.

Hi. Imo the decking is higher for a reason. Without viewing it would be impossible to give out correct advice. The best way would be to remove deck, dig out to same level as patio and a new concrete base for summer house. Otherwise wooden platform base built from deck joists will not last as long as the new timber summer house and will fail before summer house reaches the extent of its life.