Learn English – How to convert a verb to adjective

Option 1 is incorrect.

Option 2 is correct. This example uses a "split infinitive", which I think is OK. (Some people disagree with me about that.) The example would be even better with an article (or other determiner) before "user". For example:

A user can also opt to not provide shear reinforcement

or

The user can also opt to not provide shear reinforcement

The "to <verb>" phrase is English's version of an infinitive. It is often used after a helper verb, like "opt".

It is unusual to have "to <verb>ing" in a sentence. One such situation is when the word "to" is used link gerunds. For example, "I wanted to speed up, so I switched from walking to running."

It depends on the context; its position in the sentence. 1) If the word with ing comes as a subject or an object, it's a noun. eg. Smiling is an act of kindness. 2) If it comes after the auxiliary "be" it's a verb. eg. The girl is smiling. 3) If it comes before a noun, describing that noun, then it's an adjective. eg. Smiling faces are lovely.
Some words with ing are always adjectives like 'interesting' and 'exciting'.