Splenda brown is half splenda, and half real sugar. So it would not be approved. I think using regular splenda would be just as good in the spaghetti sauce, though you might wat to use a tad less than suggested and add more only if necessary. Sometimes splenda can seem sweeter than sugar.
There is a recipe for making a Brown Sugar substitute. I think it is in the recipe section in the cooking hints area. It comes from the Splenda website and is a mixture of Splenda and SF maple syrup. I also use Sugar Twin Brown sugar substitute. I also agree that you could probably use regular Splenda or leave out the sweetener.
Spaghetti squash is the only winter squash that is allowed on phase 1. You will find it in the list of phase 1 foods.