The best practice is to keep only those funds on exchange that you want to day trade or hold on mid term. For other assets and long term investments is better to use cold wallets.

It is better to store your coins in a wallet. Once a trading platform is hacked say goodbye to your beloved coins. Though having your coins in an exchange is quite advantageous too, let's say the price of the coin suddenly went up, you can easily trade it. Having it in your wallet, it will need quite some time to be confirmed and during this time price might have already dropped.
If you're in with holding the coin for a long time, store it in a wallet, and just trade it whenever you're ready or you really need to.

It depend on you are long term or short investing. If you are a long term investor i advise you should store all coin which you buy in your wallet, this will safer than. But if you are a trader, of course you have to store your coin in exchange, this will help you saving fee deposit and withdraw.

That is not a good idea,what if the exchange got hacked and then you put all your coins in an exchange wallet?you are done,that is why these exchanges their users to store big amounts of coin in their exchanges,because these exchanges are prone to the hackers.There is no safe system but you can store it in multiple wallets,hardwallets,and paper wallets are the most commonly used wallets.

The general advice is to keep your coins in your wallet but it mostly depend on your situation. If you are doing some trading stuff with this coin then you will definetely need some on the exchanges. If you don't have much coins then fees will probably be painfull for you and you will keep it on exchanges. If you are holding an altcoin and it it traded only on 1 exchange then you won't have access to it anyway if the exchange will be closed. Some exchanges have their own features that may help you to earn some intrrest (like lending in polo). But holding on exchanges always have a small risk.

If you are constantly trading, it is better to keep on the exchange, preferably at several, two factor identification will protect you. If you keep it for a long time, it's better to have a wallet. You can left your money and don't worry about that the stock exchange can be closed or something else will happen.

I do not prefer storing my precious coins into exchanges,but if i am going to trade i am putting half of my portfolio to my exchange wallet because i am a day/swing trade which needs a lot of funds but after i have gained so much using the funds in my exchange wallet i am moving it immediately so i can secured it for the long run.Exchanges are being targeted by the hackers that is why it gives you more risk of losing your money when you are putting all your funds in a single exchange.

The safest option is to keep all your coins on wallets , so much safer . And in case if the market will close the input and output of coins at the pump , it quickly can be translated into needed exchange

Neither of the two would be safe,if you want to secure your holdings or investments you need to get a hardwallet,ledger nano s or trezor would be the best choices,either of the two will protect your investments.The risk of having your investments in exchange wallet would be big because most of the hackers targets the exchanges regularly,some exchanges were hacked and all the users of these exchanges lose a lot of money so putting your coins in a hardwallet woild be the best.