A lot of people say, you are not supposed to keep all your BTC in 1 wallet, and dont do mroe then 1 transaction on 1 wallet because of security. So how does that work on Mycelium wallet?
It's actually one transaction per address. You can create a HD wallet, where Mycelium will give you one address per transaction to improve your privacy.
1) What is advantage of HD wallet? (how i have understood - Mycelium wallet will replace itself when i send my BTC on that wallet and new one will be made automaticly so my Btc transaction cant be followed) - did i understand this good or i was wrong?
Understand that wallet =/= address; An HD wallet will generate a new address after each transaction. This means that you can receive 1 BTC in X address and 2 BTC in Y address. This way people can't know that both addresses belong to the same people.
2) Why currently site for Mycelium and Mycelium wallet doesnt work?
What do you mean? This is the official website and it's working:
https://wallet.mycelium.com/3) Cold Storage in Phone App - do i need to trasnfer certain amount of BTC there, and is it only "read only" or i can use it for transactions as well?
Depends on how you build your cold storage wallet. Usually, you will create a wallet in an air gapped computer, copy the xpub or address and create a read only wallet in your online PC. This way you can see your balance and create a transaction to get signed by the private key in the air gapped pc and sent to the network through the online pc. Online PC transaction -> Send to offline PC and sign -> Send to online PC and broadcast.
4) I tried making 1 more HD Account(i have one) but all i was getting was some kind of error(i think 3 updates ago from playstore)... When i transfer money from 1 HD Wallet to another - is that same as transfering money to someones else wallet and whole transaction goes through blockchain or is something else?
When sending BTC to a new address, it will always go through the blockchain. It's like sending money to anyone else.
I know this questions may sound dumb, but i still havent decided which wallet will be "THAT WALLET". First one that i started with was Mycelium and i would really like to get more into this(planning soon to buy Ledger Nano S) and i really like that there is possibility of Mycelium connecting with it.
My suggestion is: Electrum for PC and Mycelium for android.