Applying for a fiancée visa – with a little complexity

[imb01]

Hello!

First of all, thanks to everyone for providing helpful answers so often. It's amazing that people take the time out of their day to help total strangers on the internet.

Second, my situation is as follows: I'm engaged to a man of dual British/Irish citizenship, and I'm not from the EEA. We've been putting together all the documents necessary to get a fiancée visa so I can get to the UK and we can get married there, then stay there together.
Awhile ago I found out I was potentially eligible for citizenship in a certain EEA country, and filed an application. However, the backlog there was intense – people wait for their applications to come through for years, sometimes up to 4-6 years.
Recently I found out that my application may finally be considered over the course of the next 2-4 months. Now, my fiancé and I have been apart for a few months now, and we don't want to be in different countries anymore, so we decided that despite this development, I would still apply for a fiancée visa because that would mean getting back to my beloved would be quicker (if only by a few months). We've been extremely unhappy to be apart and it's taking a toll on my nerves.

The question is this: we're applying for the fiancée visa in a few weeks. In the event that I get the visa, then go to the UK, we will schedule an appointment to give notice of marriage at the register office (probably for August, given now the notice has to take place 71 days before marriage), all of that – if throughout this process I find out that my EEA citizenship application has been successful, can I leave the UK to claim my new citizenship, and then come back without trouble? I'll still have the fiancée visa in my first passport. Will this look bad to anyone?
Given I'll be using my new EEA passport to enter the country then I should think that's okay, but then when I travel to my original homeland on my first passport, is it possible I'll run into any trouble? Is it advisable that whatever marriage date we book in the first place, we keep, even if I no longer have to apply for a spouse visa? Will I need to cancel my 6-month fiancée visa, or can I just let it expire?

So that's the complex part. Otherwise, I'd also like to know (in case my EEA passport application is unsuccessful): can I travel on the fiancée visa? (I've read online that yes, I can, up until we get married, just want to make sure).
Am I allowed to undertake voluntary/charity/unpaid work on this visa, or do internships?

And finally: just because we're really worried about rejections, I wanted to ask what would be good evidence of intent to marry (aside from correspondence with the register office, and letters of support from friends, correspondence with a photographer we'd like to hire). Are letters from family very strong proof? We might not be able to get those on time, but if they will make a big difference, we'll make the extra effort.

Thank you very much!

[CR001]

I would wait for the outcome of the EU passport application, but that is my opinion.

In saying that , you should not have an issue re-entering the UK on a EU passport if you have had a UK fiance visa. The EU passport allows free movement.

Am I allowed to undertake voluntary/charity/unpaid work on this visa, or do internships?

Work, either paid or unpaid in any form, is not permitted on a finance visa.

[imb01]

Thank you very much for responding! Much appreciated.