i send 1btc out from the wallet (from the default/first address, as i still haven't generated a new address).
You really shouldn't think of it as sending "from an address". This causes a lot of confusion in how bitcoin works and leads to people making poor decisions. Instead, think of an address as an invoice number. It is something that you can give someone to use when they pay you so that you can identify the payment when it arrives.
If you were running a business, and you sent out invoices for 100 purchases that people made from you, as you received those payments you'd check them against your list of invoices to determine who paid for what and when, but when you later spent that money to buy more equipment for your business, you wouldn't say that you were spending it from the invoice number that is was received with, would you?
I then now generate 5 new addresses (address number 2,3,4,5,6) on the same account (account number 1). And i send 5btc to each address.
Would i be able to check these 5 addresses for precisely 5btc on blockchain.info/blockexplorer
Yes.
And would that "1btc test transaction" effect the balances on the 5 additional addresses.
No, the earlier 1 BTC transaction that you have described would not have any effect on these later transactions, but that doesn't sound like the situation that your friend is concerned about.
I ask this because a friend wants a longer term cold storage solution without needing to constantly use the trezor device and instead wants to monitor his balances on blockchain. He has had problems with "change addresses" in the past with other wallets. Where address balances change freakishly when viewed on blockchain.
They don't change "freakishly". They change in exactly the way bitcoin is designed to work. You need to stop thinking of addresses as account numbers with balances, and instead think of them as invoice numbers to identify a payment
received.
When you spend a previously received payment, you spend it in its entirety. If the amount you want to send to someone is less than the sum of the previously received payments being spent, then the excess always must be sent back to your own wallet as "change" unless you want to give it to the miners as a transaction fee. Trezor should generate a new unused address to store receive this payment back into the wallet.
And also lets say i use the trezor and send 2btc out of this 25btc it holds (5 addresses x 5btc) . Which address exactly would Trezor use for the deduction of this 2btc transaction?
I'd expect Trezor to choose one of the 5 BTC payments that were received and spend it. It would send 2 BTC to the intended recipient, and would send the remaining 3 BTC back into your Trezor wallet at a new (7th) address generated for the wallet to receive the payment.
