You get rewards based on what you were at the end of the season, if you climb further (or drop) during preseason it doesn't change your rewards.
Placements are mmr based, and doesn't actually care about ranks. Your end of season mmr is the most important thing, but games in preseason do affect your mmr slightly, but not as much as during regular season (I believe it's half as much but don't quote me on that). So playing the same number of games with the same win/losses in preseason will have less effect on your mmr that regular season ones, and so while you might make diamond 5, your mmr would be lower than if you made diamond 5 in regular season. Then the ranked reset pulls your mmr down a bunch, and then you do placements and get placed based on whatever mmr you have at the end of everything.
Preseason will probably last until sometimes in january.
If you drop from plat to gold before season ends, you'll receive gold rewards instead of plat ones. If you're plat at the end of the season, and drop to gold before you actually get your rewards, you'll still get plat rewards, since it only cares what you were at the end of the season.