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I consider making a Nutella-like spread on my own, seeing how it would be much cheaper and also could be made to contain more hazelnuts and less (non-palm) vegetable oil or sugar.

Looking at Nutella's ingredients, it contains no preservative: sugar, vegetable oil (probably palm), hazelnut (13%), skimmed milk powder (8.7%), low fat cocoa powder (7.4%), soy lecithin, flavoring (probably synthetic vanillin). 30.9% fat, 56.3% sugars.

After opening it doesn't seem to mold or anything even after several months outside of the fridge.

Yet someone asked a similar question here about their hazelnut spread going bad within weeks. The answers weren't that clear.

The person said they added a small amount of hot water to the mixture, and it isn't mentioned the mixture was heated at any point. Some people commented that might've been the culprit, but beyond mold the risk for botulism was also mentioned.

So my question is whether using an almost-exact replication of the Nutella ingredients, no water and heating it up to a certain (?) degree (plus storing in a sterilized container) could promise long-term consumption safety? Is there a minimum % of sugar recommended? And is botulism an actual risk with such a product?

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In short: your recipe for a heated hazelnut spread is safe.

The previous question and answers did not address the effect of water activity (Aw) on microorganism growth and safety. The added water in conjunction with the sugar and added protein were the root cause of mould growth in that scenario.

In most foods, the water present may be bound to components within the food like salts and sugars, making less of it available for use by microorganisms. The amount of available water, Aw, is a measurement of how much water is not bound - typically, a value below 0.92 inhibits Clostridium botulinum growth, and below 0.85 most organisms including yeasts and moulds will not grow at all, making the food shelf-stable. This is just for stopping growth, and does not account for other effects in the food - i.e. stress or lethality from osmotic or acid stress.

The presence of Salmonella on raw hazelnuts is documented and cited by the US FDA, though at a lower prevalence than for other tree nuts [1] - roasted and other processed forms are not included.
Nutella relies on the effects of water activity to prevent pathogen/spoilage growth and survival after a lethality cook process from roasting. The added dry ingredients and hazelnuts themselves start at a water activity less than 0.85 - some articles cite a range below this for quality purposes to inhibit lipid oxidation [2,3], and the Oregon Hazelnut Industry claims a range of 0.46-0.49 for dry roasted products. Under the ideal drying and storage conditions for hazelnuts, Salmonella viability decreases as well [4].

Salmonella in tree nuts

Findings of Salmonella in raw tree nuts. From:
"Prevalence of Salmonella in Cashews, Hazelnuts, Macadamia Nuts, Pecans, Pine Nuts, and Walnuts in the United States". [1]

Water activity of hazelnut varieties

Physical characteristics of hazelnuts. From:
"The hygroscopic behaviour of the hazelnut". [3]

[1] Prevalence of Salmonella in Cashews, Hazelnuts, Macadamia Nuts, Pecans, Pine Nuts, and Walnuts in the United States.
Guodong Zhang, Lijun Hu, David Melka, Hua Wang, Anna Laasri, Eric W Brown, Errol Strain, Marc Allard, Vincent K Bunning, Steven M Musser, Rhoma Johnson, Sofia Santillana Farakos, Virginia N Scott, Régis Pouillot, Jane M Van Doren, Thomas S Hammack.
https://doi.org/10.4315/0362-028x.jfp-16-396

[2] Effect of drying methods on long term storage of hazelnut.
Ali TURAN, KARAOSMANOĞLU.
https://doi.org/10.1590/fst.20518

[3] The hygroscopic behaviour of the hazelnut.
A.Lopez, M.T.Pique, M.Clop, J.Tasias, A.Romero, J.Boatella, J.Garcia. https://doi.org/10.1016/0260-8774(94)00021-Z

[4] Salmonella Survival Kinetics on Pecans, Hazelnuts, and Pine Nuts at Various Water Activities and Temperatures.
Sofia M Santillana Farakos, Régis Pouillot, Susanne E Keller.
https://doi.org/10.4315/0362-028x.jfp-16-392

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