Only for wooden sleepers. For concrete, you need dual gauge sleepers (not gauge convertible ones).
All wooden sleepers are gauge convertible, that's not the case for concrete sleepers.
Irrelevant for freight traffic. As for passenger traffic, with adequate planning you can gauge convert the rollingstock in about the same amount of time it takes to gauge convert a railway line. See: V/Line Albury service.
How could it be relevant for passenger traffic but not freight traffic? As for passenger traffic; Remember that regauging a large network, if feasible, must be done bit-by-bit, as re-electrification is.
Fleet renewals are almost certainly done with a phased approach, being done as new rolling stock becomes available. During fleet renewals, there is a time period where the old and new vehicles share tracks. If the line is being regauged at the same time as the fleet is renewed, this would thus involve dual gauge track.
You get all of the disadvantages of DC electrification (bulky rectifier substations), without any commensurate benefits. The benefit of 25kVAC et al. electrification over lower voltage (600-3kV) DC is that the trackside infrastructure has a much lighter footprint.
No, you do not get the disadvantages of a lower voltage D.C system. The span between substations is equal to or maybe greater than the with A.C.
The benefits of A.C, when A.C electrification was introduced, were as follows; The line voltage could be higher that the operating voltage of the motors as A.C voltage can be stepped up and down just with a transformer. Additionally, the power supply to the motors could be controlled with a tap-changer rather than a combination of resistors and series-parallel switching, both now replaced by switched-mode motor controllers.
With DC-DC converters rather than plain old transformers to step down the voltage, 30kv D.C would combine same benefit over a low voltage (550v-3kV) DC as 25kV AC, with the advantages of low voltage D.C.
As for the 'benefits' of your idea... power delivery has never been a problem for 25kVAC systems. High Speed Rail trains consume as much as 25MW each - and they don't pose a problem for the electrification system.
Phase breaks? They aren't the problem you seem to think they are. I know I'm talking to a brick wall when it comes to that area of discussion though.
I didn't say power delivery was a problem for either A.C or D.C systems, but D.C delivers more power than single phase if the peak voltage of both are equal. The value of current, whether direct or alternating, that produces a given amount of power dissipation in a resistive load is pretty much the weighted average. With D.C, the weighted average (or root mean squared - R.M.S) is the same
as the peak voltage. Given an A.C voltage of 25kv R.M.S, the peak voltage is approximately 35,3551
An A.C electrified network, with power supplied from the national grid, has at least three sections on different phases, or alternatively, could conceivably have two sections on different phases if the three phase is converted into two phase before distribution. Phase breaks are needed between different sections on different phases and this involves a neutral section, power supply to the train being interrupted during this.
A D.C electrified network with only one substation, doesn't need neutral sections at all.