1. Dear Sigrid, at the outset, i would recommend that you get a second passport from your home country in order to be able to travel whilst your first passport is being processed by the Home Office when you are applying for naturalisation. Both Irish and British authorities allow a person to hold a second passport as it is commonly necessary where someone is visiting, say Israel and some of the Arab countries successively.
2. However, what you say concerning naturalisation as a British citizen is correct. You will need to hold a Permanent Residence card for a period of one year, before you can apply on the basis of five years continuous residence in the UK. Here is a link to the relevant application form https://www.gov.uk/government/publications/apply-for-a-document-certifying-permanent-residence-or-permanent-residence-card-form-eea-pr.
3. However, as a general principle, unless you apply for Indefinite Leave to Remain, you will need British citizenship in order to definitely continue to live in the UK without any trouble. There will be some review of the legislation in this area. However, it is difficult to predict with confidence what will be the outcome post-Brexit.
1. Indefinite Leave to Remain is what non-EU nationals apply for. It is essentially the same as getting a Permanent Residence card. However, at the moment, getting Indefinite Leave to Remain gives a life long ability to live and work in the UK whilst this is not automatic for EEA citizens as they are taken to be able to come and go as they please and work if they want.
2. Indefinite Leave to Remain is a possibility. However, be aware that the requirements are similar to citizenship. So, I am not sure there is any better answer than citizenship. ILR is a halfway house, but it does guarantee you the right to live and work in the UK INDEFINITELY.