It goes to ur collection in ur bank. So if u do dismantle something and want it later u can get it back for some legendary shards the downside is u have to have gear to infuse it with to bring it up to your power level
No. You can re-buy it from your Vault 'Collections' tab any time for a small quantity of exotic shards. Once you earn any exotic, it's essentially unlocked for all toons on your account.