Being asked to take on security responsibilities aligned with prior career but outside my current role and pay - how should I handle this?

Your (most optimistic) bet is to go along with something like:

Hello (manager). I changed my job and joined (company) because I did not want to work in cybersecurity anymore, because I want to focus on (technologies). This is why I explained you from the beginning, even during the interviews, these details.

Of course, I hope you told them you do not want to work in cybersecurity.

Your reality seems to be a bit less than optimistic. They already denied adequate payment for the cybersecurity senior role you are (kind of) forced to undertake. In this situation, you need to analyze you options and decide what to do next:

• take it; accept their unfair proposal;
• leave it; start polishing your CV.

Your advantage during the negotiations is this:

I accepted a lower-level position and compensation accordingly, viewing it as a career reset toward more technical work.

So if they want you to return to your previous work, they have also to pay you according to your previous work. And the compensation cannot be lower than that, but at least several percents higher (nobody leaves a job for less payment, for the same work).

You can also point out that:

I'm also aware that the company's current security posture is well below the certification's requirements, so this would not be a small or quick task.

And explain the complexity of the job expected to be done - which needs an even-more increased level of compensation. I.e., point out several of their smaller problems, to make your point. You do not want to give them high quality knowledge for free.

I was told that security falls under the scope of a cloud engineer ...

So does the cleaning of the dust from the servers. Does the cleaning of the dust have the same payment? If yes, take that job.

... that I'm the best person for this work right now ...

That automatically qualifies you for a senior position, with suitable compensation.

... and that a dedicated security engineer might be hired later if needed.

A qualified security engineer is already needed, that is why they want you to do the job. They should already hire now, not later. And if you leave the company, they will have to hire one anyway. At least now they know and trust your abilities, but they have no idea what to expect during an eventual job hunt.

I'm several years away from returning to a senior level

No, you are not. On topics of cybersecurity, you already are a senior. Actually, you already were a senior even before your current job.