Hint for the first question:

An argument scheme being valid means that all instances of sentences of this form are valid; if the form is invalid, then not all instances are valid. According to this definition, could it be the case that there exist valid instances of an invalid form?

Hint for the second question:

An argument is valid iff in all structures, either at least of the premises is false or the conclusion is true, and invalid iff there exists at least one structure (a counter model) under which all premises are true but the conclusion is false. If the premises are inconsistent, i.e. true in no possible structure, can there be such a counter model that makes the premises true and the conclusion false?

The technique of counter example is what you first question proves. That is if your argument is valid then I can change the content to any topic I desire with the same form and my new argument must ALSO BE VALID regardless of the subject matter. Think about how often something can be true in one subject but false elsewhere. It can happen and often. One counter example proves the truth is not 100 percent. At best you have a half truth. The 2nd question is confusing. Yes you can have an invalid argument with inconsistent premises but not sure that is what you really mean. Clarify. – Logikal – 2020-08-23T02:43:15.597