Mars continues to have large amounts of water ice in the form of sub-surface permafrost across most of the planet.

Mars had water far more recently than it had any substantially different orbit. The sun has been getting brighter over the past four billion years, too, but afaik that isn't enough to stick together a full Martian climate history. There's other stuff going on which we don't fully understand yet.

Opportunity is far more impressive than Voyager. Voyager has basically been sitting in a nice clean vacuum doing nothing for the past thirty-five years, it's no wonder it's still working. Opportunity has been hard-scrabbling around in sandy, dusty environments, heating up in the day and cooling down at night, with dozens of joints and moving parts waving around all day.

I was under the impression the accepted reason for Mars losing water was it being carried off by solar wind due to Mars' lack of a strong magnetic field, that lack in turn being caused by its smaller size making its outer core not remaining molten like Earth's to generate a dynamo current.