Is it necessary to register a gite business?

I am a resident in France and have lived here for 6 years with my wife. I will be 63 in June and my wife is 62. We have decided to rent our main residence as a holiday home for 11 weeks this summer and have registered it with a large UK holiday company. I am unsure of the best method for the registration of the business, if I should register at all. I have already registered with my local mairie as I know this is compulsory. I had thought auto-entrepeneur would be best but I am unsure now. Let me detail my situation and concerns. I will operate the business for 2 years and if it is successful for up to 5 years. In this first year the business has the potential to generate up to €18,000 but I expect it to be much less, and perhaps no more than €5,000. In the second and subsequent years income could be in the region of €25,000 but again it is like to be much less than this figure.

My wife has a UK state pension because of her age. I have a UK government pension, taxed in the UK and exempt from French tax, and I also have a UK private pension. I would expect the gite income to be less than our current income. My wife and I have been affiliated to CMU since our arrival in France in 2007 and contributed up until 2012, when my wife registered her Form S1 as a UK pensioner. I was included as a dependent so remain afflicted to the CMU but without payment.

1. I know that as an auto-entrepeneur I will be contributing to a French pension. I have read that, when I reach French pension age, all my income will become subject to cositations because I am part of the French pension system. Is this true or can you elaborate please?
2. Is it necessary to register at all, other than with the mairie?
3. If I do not register, am I right in thinking I declare my income on Form 2042?

Initially, I would like your advice about which regime to adopt for my gite business.