Pokemon-go – Can you use the effects of several berries at the same time

Possibly, but only Niantic knows when. I would strongly discourage waiting for it.

The thing about usernames and gaming is that you need more than confirmation that the original account has been closed. You need the developers to release the actual username.

Until Niantic actually release the username, their servers will still register it as taken, even though it points to a 'blank' user. Companies have been known to release usernames, in the past. That said, it is often the result of years and years of activity, where the game has run for so long that there would be a large number of 'deleted' accounts, and usernames that could be recycled.

There are two options available to you, in terms of obtaining the user name again, in the short term.

Ask Niantic

If you contact the developers directly, there is no guarantee, but they might very well give you the ability to 'pass on' your username. Be cordial, and ask them nicely, as opposed to demanding they do it. Enforce that the username currently belongs to your account, that you would like to give it to your friend, and that the new username would make their experience more enjoyable.

Ultimately, developers are always more capable of acting if they think it would make a bigger difference to their users, but much more goes into play. Do not be surprised if they reply to say 'Sorry, but such action is outside of our capability' or 'Sorry, but that would not be fair to other users'.

Give your friend your account

This section had previously detailed options for giving your friend your account. That said, Niantic has since updated their FAQ to enforce that this practice is considered cheating by their standards, and will result in account termination, so it probably is not the best course of action.

Like Samthere said, you can delay indefinitely, however id recommend keeping one Pokemon high to take gyms with. Don't power it up more than you have to but keep it high enough to get at least a few gyms. The extra star dust and coins you get should be worth the candies and stardust you spend to keep just one Pokemon high. I usually get rid of anything with a CP meter under 90%. It's probably good to set a goal like level 25, otherwise you probably will delay it indefinitely. Lastly, go ahead and evolve any Ratatas and Spearows (or other really common ones in your area) you can for the experience. Their evolved forms max out pretty low so they aren't really viable later, otherwise leveling will get really slow unless you are hatching eggs left and right.