What's the purpose of the support? A windmill needs to be supported only in one place, at the centre tile. The support isn't needed to hold up the windmill.
If you want to run rollers from the windmill, you must use a gear assembly to support it. You cannot switch power via the supporting gear assembly itself, that'll cause the windmill to deconstruct. You have to either put another gear assembly between the 'holding' gearbox and the rollers _or_ attach switchable 'load' to the rollers, so you can switch the machine off by overtaxing the system. E.g. if you have 20 power from the windmill, you can run a gearbox and seven rollers off it - that costs 19 power; when you attach another gear assembly to the system, that will increase the power consumption to 24, bringing the entire system to a stop; but if you switch that 'load' assembly off, it will no longer require power and the rollers will start up again.
When trying to smash foes with speeding minecarts, it's really necessary to build a construction that can hit a target repeatedly. A minecart hit will easily break bones, but to actually kill you can easily need a dozen or more hits.
I was feeling rather vindictive after an axegobbo squad uncloaked right on top of a hauler, so released four captured hammerers on the wrong side of my minecart curtain. The basic construction is breathtakingly simple:
WRRRRRRW
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'R'ollers in the north push south, the southern rollers push north. 'W'alls to limit access to the rollers themselves (in case building destroyers happen to my fort - to get into disassembling range, they have to step on the track). Each roller pair bounces one cart back and forth between them when active. When the rollers are switched off, carts stop on the nearest roller. With six roller pairs (and minecarts), that's a mere 24 power to run. I'm consuming 105 power total, so the most is actually spent generating and transporting it.
The quickest goblins managed to reach the second track, with no hope of actually passing all six. All carts are made of metal, ranging from nickel silver to gold. A dozen hits are generally enough to cause lethal damage - either a brain bash or disruption of the upper spinal chord (guaranteed suffocation for breathing units). It appears to be brutally effective against anything goblin-sized, but i don't have larger stuff on hand to test how it fares against, say, horses or trolls.
As an aside: the 'machinery freezes in cold climes' rule seems to be restricted to contact with natural ice floors. The machinery works without trouble on _built_ ice floors, but once any 'frozen here' item was included in the drive train, things get strange. Removing and rebuilding the frozen item doesn't restore mobility, the train will still list as 'frozen elsewhere' until you destroy/disengage a _non-frozen_ construction; however, doing so can restore mobility to 'frozen here' items, i.e. stuff that's actually sitting on a natural ice floor and should remain frozen.