1) do I need keep my wallet online when I send/ receiving btc and wait until the confirmation is finish.
no, only when you want to send from that wallet and only when you want to check the new balanceQuote
I once move my btc from blockchain.i to coinbase it took almost 3 days to finish the whole transaction (I set the transaction to the lowest fee (365 satoshi) before I made the transaction 
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Im asking this because I have limited internet connection, I cant go online for 24 hour or more. that woud be impossible to wait that long.
as i said you dont need to online )Im asking this because I have limited internet connection, I cant go online for 24 hour or more. that woud be impossible to wait that long.
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And If yes we have to go online on transaction, isnt it pretty risky for doin that?
2) What happen if someone send me bitcoin but my wallet is offline. lets say Im not online for 2 weeks
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will the coin arrived after I go online and the wallet synchronized or not. And what happen if Im the sender.
your wallet just handles your transactions.2) What happen if someone send me bitcoin but my wallet is offline. lets say Im not online for 2 weeks
! will the coin arrived after I go online and the wallet synchronized or not. And what happen if Im the sender.
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3) What happen if Im in the middle of transaction (either sending/ receiving) but then I get disconnected from the internet, or have my pc freeze. will it affecting to my balance, will it affecting to the transaction value i.e wrong transaction value or etc.
you can always check your balance, transactions and other stuff with online block explorers to see what is happening.Quote
4) I see that electrum & multibit (and perhaps others liteweight offline wallet) did not download the whole btccore, so that mean my btc is floating around on bitcoin network or internet, but it is locked by my private keys (is my statement here correct or not)
5) I know its silly question, but what happen if electrum/ multibit stopping their services, closing down their servers, is my btc will gone?
as long as you have your "private keys" you are safe. which means you can import them in any other service and use your bitcoins5) I know its silly question, but what happen if electrum/ multibit stopping their services, closing down their servers, is my btc will gone?
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6) Is it wise to use 2 diffrent wallet client on same machine? I mean if 1 client is compromised/ corrupted will it affect to the other client?
7) I have 2mbps internet connection, yes its slow, will it affecting to my wallet performance or transaction?
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what you send and receive with your wallet is not that much especially since you are using a lightweight wallet. so no