Once you have delegated your GRT, you are contributing to the network's security. In return, you will receive rewards in the form of newly minted GRT. Conversely, if you do not participate in staking, your assets will get diluted over time.
The indexer you delegate your tokens to is not able to access your funds at any point in time since you are only delegating your staking rights. At no point in time, we have control over your GRT or your private key.
Rewards are either distributed to delegators every time an indexer closes his/her allocations or they are automatically distributed every 28 days.
Your GRT rewards accrue on your account and are automatically re-delegated, so that you can leverage a compounding effect on your funds. Rewards are distributed to delegators everytime a indexer closes his/her allocations.
You need to stop the delegation, wait for the unlock period to complete (~28 days), and then withdraw your GRT. Please note that when you are withdrawing your tokens, you have to pay a withdrawal tax. The tax is in place to disincentivize poor decision making that could harm the integrity of the network. You can choose to undelegate any amount of GRT, so you do not need to undelegate the whole amount.
You can simply delegate more GRT to your indexer via the Network Beta dApp.
Unfortunately we cannot answer this question in a general manner as tax regulation differs among legislations. Nevertheless, we advice you to always track your staking operations so you can provide a detailed history of your staking rewards.