The Mycelium fee rates are indeed "dynamic"... So, the rates used by the various options will change depending on what is currently happening with the network. So, you're kinda stuck dealing with whatever Mycelium's current "guess" is for 'low-priority', 'economy', 'normal' and 'priority'.
The devs seem more interested in milking the app for whatever gains they can by force feeding user's ads, implementing dodgy ICOs and getting affiliate fees from credit card enabled coin exchanges that have outrageous currency spreads.
It took immense amounts of moaning and bitching by (Android) users to get the scrollable options for the fee rates...
It appears that this setting is not available on iOS either. But I do know that my addresses are legacy, and at a first glance I think I can only have one per wallet, unless mycelium creates another address for each transaction I'm about to make. There's no feature for generating more addresses either from what I can see, but at least they give me the xpub of the address. I'm not sure how useful that will be though.
Mycelium is indeed an HD wallet, so it will generate multiple addresses for each "account" that you have... However, like a lot of HD wallets, it doesn't allow you to create a new address until the previous address has been used... in addition, it will do this automatically.
So, you click "receive" it'll show you Address
A... when it detects that Address
A has received some funds, the next time you click "receive", it will automatically show you Address
B and so on...
I'm guessing my address is at m/44'/0'/0'.
The good news is that Mycelium is BIP39/BIP44 compliant... so yes, your addresses will start at m/44'/0'/0'/0/0 and increment from there... You should also have the 12 word recovery phrase that you can use to migrate to another BIP39 compliant wallet if required.