Just having a quick think here....your "push" gravity between two "particles" in this instance (and in all instances) is in violation of the 3rd Law of Thermodynamics i.e. the order of a system, over time, will collapse into complete randomness...the entropy of the system increases over time.
In which case, the ordered system of a gravity "shield" would collapse and the random push of the "ultramundane particles" would dominate. The shielding requires that an ordered system of "less gravity or less ultramundane particles" be present between the two objects and is sustainable long enough for those particles to attract one another. Just through simple energy exchange between the particles and the radiation of that energy to their surroundings, the entropy in the gap would increase....and if given LeSage's quoted speed for these particles, almost instantly. There'd be no shielding effect that would last long enough to attract particles together. In this case, the universe would fly apart.
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