I suspect there may be some confusion (at least on my part!) regarding what we mean when we say space and time are absolute for the Leibniz type philosophical argument and the meaning of absolute used in the Einstein space/time sense. I think we possibly have two different types of absolute / relative distinctions going on.
For Einstein the argument is whether time is "fixed" and this sense of absolute is different. Einstein says no, time is relative to the observer.
For Leibniz the argument is whether time is absolute, i.e. exists independently of events that happen.
I don't think these two meanings of absolute relative are quite the same, but am happy to stand corrected as it is a fairly mind boggling subject!
If time is relative to the speed of light (which I accept) then could we not still say that time is absolute as far as Leibniz debate is concerned? If the speed of light is fixed, then in one sense time is also fixed in relation to the speed of light. Time is therefore the same everywhere, allowing for its relativity to the speed of light. Time is not something which is imposed by the mind, it is a real "thing" and depends on the speed of light and distance (i.e. not on events).
Similarly for space. Space may be able to bend, but it is still and absolute "thing" in one sense, again relative to the speed of light and time.
One interesting question, which I don't profess to know how to answer, would be will space still exist if there is nothing in the universe (i.e. the heat death of the universe). Once entropy has worked its wicked way to total finality, could we still talk about space?