So pleased you love your new cut!
I never heard of natural oils lightening hair so I can't tell you how to avoid it. Sunlight absolutely can lighten hair and is very damaging, and some ingredients can accelerate that process but it's the UV doing the lightening and the damage. Are you reading the ingredients? There is very little natural oil in Moroccanoil products anyway, they contain all sorts of lab made chemicals.
Flat ironing has to be done on bone dry hair or you risk 'bubble hair', I would be wary of a heat protectant that has water as the first ingredient unless you intend to wait a good while after application for complete drying. The alternative is an alcohol based spray which can be drying, or a cream/ serum protectant - I can't recommend one, flat ironing kills my hair.
Hair then has to stay bone dry or the hair will revert (wave/ curl will come back) so you cannot moisturise - add or increase water of - newly flat ironed hair. Products that are used afterwards are generally silicone rich serums that give faux shine and slip and act as anti humectants, that is they keep water (moisture) out.
Sorry I'd love to tell you what products will give you healthy hydrated heat styled hair but AFAIK
that doesn't exist. Ingredients that protect in a limited way include hydrolysed protein, certain polyquats as well as the silicones. Check out YouTube for wet wrapping tutorials.