The Freestyles and the Debuts are both fabricated on the Jackson Elite last (boot form). Therefore the fit will be very similar, though not identical. Differences in construction elements such as padding, lining, and tongue will affect the final fit. The basic fit should be close enough, however, such that if the Freestyle fits well, the Debut should also fit well. On the flip side, if the Freestyle does not fit well, the Debut won't either.
The Debuts and Freestyles both come in two values of stiffness for women's and two values of stiffness for men's. Debuts are available in stiffness values of 45, 50, 55. Freestyles are available in stiffness values of 40, 45, 49. So they aren't waaay different, and you should get a reasonable feel for stiffness from trying on a Freestyle. Worst case scenario would be if the shop has only a 40 women's Freestyle for you to try on, but you decide to order a 55 men's Debut.
You've got the sequence in reverse order. As much as feasible, adjustments to the boot (such as heat molding and punching out) should be viewed as fine-tuning operations, rather than major surgery. So, you should first try on a variety of boots (which under your circumstances might be limited) to see which provides the best fit as-is, and then consult with the tech to determine whether it can be adjusted to resolve any remaining issues. Some issues may not be able to be resolved in a stock boot.
If two models are close, the tiebreaker would be to consider which is easier to adjust and which has a wider range of adjustment. Different models (even by the same manufacturer) might be fabricated from different materials and might have different construction details. For all this, however, you are dependent on a competent, experienced tech ... so ask your coaches and more experienced skaters for their experiences.
Again, check your shop's return policies. In the US, typically if the boots are modified in any way (including heat molding and mounting blades), they can't be returned. If they aren't modified, they usually can be returned, but also usually with a fee. Policies vary with individual shops (and often depends on whether you're a new customer or a long-time customer who's done a lot of repeat business and brought in many referrals). So ask up front.