Expenses, social charges and costs for gite and furniture business?

As mentioned previously, I am about to register as a gite owner in France and my partner for selling furniture.

1. We are installing a new pool for people staying in the house and we’re also installing a new fosse septique to conform. Under auto-entrepreneur I couldn’t claim these expenses. If I were to set up under régime réel simplifié, do you know if either of these would be deductible expenses? I’ve read somewhere online that the pool wouldn’t be. Also, how are social charges calculated on such a scheme, if it allowed for such items and I showed a loss in year 1?
2. Once the furniture business is set up, it might be better set up as régime réel simplifié with stock, rent and shipping costs. Can you add on additional professions - as with auto-entrepreneur?
3. Is it legal/acceptable to use a Centre de Gestion Agréé as an alternative to an accountant on régime réel simplifié? Alternatively could an accountant be used in year 1 and then move to a CGA in year 2?
4. Under micro-bic, say I paid 2900€ in social charges in year 1 on a turnover of 12,000€ on the gite, I think that social charges would usually be 3480€ x 46% = 1600€. Would such an overpayment of 1300€ be carried over to year 2 (or refunded) or is 2900€ the basic minimum one pays per year?
5. Regarding section 7 on your video about auto-entrepreneur registration and adding my partner onto my assurance maladie, he is currently ayant droit as personne vivant maritalement. If I include him here, does this mean I’ll be charged 3000€ in social charges or would that only be incurred by adding him earlier in the form?