Elongating the slots would cause a weak area in the plate, right where it's subjected to the greatest bending loads. 1/4 T1 steel is very strong stuff, and statically could easily take the load, but because the centripetal loads are not in line with the T1 plate, I'd imagine it would fail within a few hundred thousand cycles, give or take. Mostly because the T1 plate isn't lined up with the strain caused by the blades pulling as they swing around and that stress wouldn't be constant as in the lower part of the cycle gravity works with the centripedal load, then in the uppper part of the cycle it works against it, if the assembly isn't stiff enough to resist that bending, the it will fatigue. T1 sounds like a reasonable choice for a high fatigue assembly but the simplest thing would be to make a second plate, it need not be T1, and drill it exactly how you want it (to match the proper hole spacing in the other plate), then use it on the front of the assembly, both to evenly compress the wood blade roots, and to minimize (effectively eliminate) bending of the mounting plate. Done perfectly it would probably also include gluing in bushings in the wood blade hubs, that the plates would compress too. I doubt you would get much benefit from the bushings, but am sure you'd get a lot of security of mind from a front plate. Heck, you could even make it out of plywood or aluminum, and it would still probably result in an assembly lifetime of 100 times what you are looking at now.
