How do I raise interest in a different internal role without looking uncommitted?

How I view this depends a lot on the company's culture. I've worked for four years prior to answering this, and I'm fortunate enough that the company holds this very positive principle: The interest of the company and the interest of the employee should go hand-in-hand. I was also fortunate that to all the three managers that I have reported to, speaking openly with them my aspirations and concerns never backfires to me.

I would say the best thing to do is communicate openly with your manager, just as you wrote it down here:

"I'm genuinely interested to go for the normal career path, but the new developmental role opportunities for me sounds like a good fit for my growth as well. Would it be possible if this can be set as my stepping stone? In the end, my target is still the normal career path, but I believe this role could prepare me better"

His/her response might be that it wouldn't work, or there's a work around, but I feel like this is the best way that you can find the resolution to your concern here.

For some context, I'm new to the company with 6 months experience under my belt, and am currently on a 12 month FTC, looking to secure a permanent position.

This is the most important part. You need to know how this potential switch will impact your getting a permanent position. It could be good if the switch will get you a permanent role, or it could be a disaster if that means that switching now will mean that your contract will end without an extension or a permanent role.

In addition, some companies block the ability of new employees to switch for x months. They do that because the team that hired you expended resources to fill the position, and if they let people switch too early they never get a positive benefit before having to start the process over again. Find out what is the policy for your company.