It shouldn't be a problem to receive more than one payment to that same address, since all your old addresses are monitored for payments. There might also be an issue that, since HD wallet addresses are generated sequentially, the wallet scans each address to see if it's empty, and if it is, assumes that it was the last one used and doesn't scan for any new ones (we scan 2 ahead). So there may be an issue where, for example, you give three people an address to send to, each one having a new address, and the first two people don't send anything. Since those addresses will be empty, the wallet will think that it's the end of the used addresses and that there are no payments to any later addresses, and the third person's payment (or any after it) will not show up. Only way to get around that is to scan ahead for even more addresses, and/or limit the amount of empty addresses you can generate. But as I said, there shouldn't be a problem with two people sending to the same address.
Should we manage to implement stealth address payments, this won't be a problem anyway.

I was playing around with the new version and created a 2nd account (from the same HD seed, I assume). It was just a test, I don't use this 2nd account, and it has no transactions. How do I remove it?

Do I delete the app data and restore from the backup?

Still trying to wrap my head around HD wallets.  When I use Mycelium for sending and receiving funds, does it automatically ensure that I use private keys only once per addess for signing transactions?  In other words, does Mycelium manage the creation and use of addresses so that I never reuse one? (Even for receiving funds?)
Thanks

pm7 explained it, but I'll just add that you can't prevent the reuse of an address.
Let's say you use mycelium HD wallet and Joe Blow sent you some bitcoins to an address it generated.  Now that he has that address, there's nothing to stop him from sending coins to it a second time, and again after that.  This means the wallet has to remember all the keys and addresses it has generated.  Of course, an HD wallet can simply regenerate all of them from the seed, but regenerating the wallet and syncing it with the blockchain would take time, so I'm sure mycelium doesn't that every time you open the wallet.  I don't know the exact method it uses to keep track, but there's a lot of information in the two mycelium threads in this subforum, and mycelium hews pretty closely to the HD bips, if you want to read about it there in the bip32 and bip44 mediawikis.
This is the mycelium HD thread:
https://bt.irlbtc.com/view/814907.0

Ah, yeah, on that topic again: what is "fast" in your case? Like I said I'm looking at 20+ sec with 25 private keys in HD and 3 "legacy" keys. Every time I start the app. And about 10+ sec every time I access the "Transactions" tab.

Internet is good in general. It's a Samsung Galaxy Grand GT-I9082 - certainly not the fastest horse in the stable, but still ok, I would think. It definitely changed after the 2.0 release, before that I had sync times similar to yours (same phone, same Internet).

My earlier post, in case you missed it: https://bt.irlbtc.com/view/293472.msg9152515#msg9152515

I assume Mycelium uses the "account" level from BIP44:

So in other words, separate HD accounts are like different bank accounts owned by a single entity with a single (backup) password.

Bitpay is the first to utilize NFC + Bitcoin app and POS...

I have been talking about this for over 1 month, NFC, signing and broadcasting would look great in Mycellium app

And while we're at it, would it also be possible to spend unconfirmed money? Some other wallets do this as well, including the very popular Bread Wallet on iOS (by far the best iOS wallet in my opinion).

Right now, if someone sends me money, Mycelium shows "receiving xxx BTC" but I have to wait until it gets confirmed, before I can spend it. Yet technically, it's absolutely fine to sign transactions with this unconfirmed money and spend it right away. I have tested this in practice and it always works without problems, as is also my experience with bread wallet on iOS.