There is no “best” direction to walk the Speyside Way: it depends on whether you want to follow the river downstream from mountain to sea or be like a returning salmon and go upstream. Some practical pros and cons are mentioned here.
For most people the answer will be 6-8 days depending on route choices: see this page.
You can walk it at any time of year, but in winter, be prepared for short hours of daylight and boggier ground underfoot. Whenever you visit Speyside remember to be prepared for any weather: see Outdoor info.
In summer months, both midges and ticks are possible, though less likely on Speyside than in many parts of Scotland. Read the section about this under Outdoor info.
No: the route is named after the Speyside area, not the river itself. Although you will see the River Spey’s estuary, walk beside it in places and cross it many times, this is not a riverside walk. The Spey valley has wonderful scenery, supports a wide range of wildlife, some of it rare or threatened and is famous for its salmon fishing and whisky distilleries.