Can a reference list for a research paper contains references for resources that are never cited in the text but was useful for the research?
A reference list is the list of things that are referenced. A list containing other items that were used is called a bibliography.
No, a reference list only provides the list of references that were cited in the main text. If additional literature was useful for the research, it should be cited accordingly. Unlike a syllabus, a reference list is not just a collection of literature on a certain topic.
No, it can't. This is actually checked during the proofreading step and you are asked to remove all items from the reference list that have not been cited in the text. That is actually pretty common to happen (as is the opposite of missing items in the reference list) since it's easy for citations and reference list to be out of sync if you don't use a reference manager and have a lot of revisions.
I think it's technically ok, but It's defininitely bad style. How was it useful and yet not worth refering to in your manuscript? It also leaves unclear which part of your research it supported.
Elsevier e.g. actually state in their author guidelines that cited references must be mentioned in the text and vice versa: https://www.elsevier.com/journals/learning-and-instruction/0959-4752/guide-for-authors#68000