They should be completely winter cold hardy anywhere in zone 7. However, the mix you filled your raised beds with is not ideal for lavenders - they prefer a very lean and fast draining soil and I am concerned that ALL your ingredients are far too moisture retentive, given most z7 locations in this country tend to be pretty wet in winter.

That would be the limiting factor IMO - not the cold, just excessively soggy winter soil.

Do you have any suggestions for ammending the soil around the plants to help with that? I can "replant" them right in place if I need to? Vermiculite? Sand?

As long as it drains well, plain old garden soil is just fine. Vermiculite does not really help with drainage. In fact, once it becomes fully saturated with water, it collapses and any voids or pore spaces that might have helped with drainage are lost. It really has no place in an outdoor garden soil (and one of my major complaints about SFG and Mel's mix). And you would have to add a very large volume of coarse sand to offset the effects of the compost and peat.

If well draining existing garden soil is not available, then I'd add a quantity of bark fines.

Also bear in mind that peat, besides being water retentive (or, when completely dry, the opposite problem - hydrophobic!) it contains zero nutrients and is acidic. Lavender prefers a rather alkaline situation. I'd mix a good handful of crushed eggshells into the soil under/around each plant, maybe even some coarse (not pea) gravel.

But I certainly agree one must be more concerned with a well-drained site than for your zone for lavender over winter -- this is assuming you have a hardy English lavender and not Spanish/French lavender -Lavandula stoechas, although that may be OK in zone 7, but 8-9 better. My Munstead lavender does OK in zone 5.