You're on the right track and sounds like you've gotten some very good advice, which I'll second (or third). The tarp you've got will work in a variety of conditions. I've got the same tarp. Though I no longer use it backpacking because of weight, I did so for several years and found it to be a tough and economical choice that never let me down.
Based on the scenarios you presented, I agree with others that your best bang for the buck would be a synthetic underquilt. I too have an AHE Jarbidge and can recommend it.
If lows are in the upper 60s to low 70s, the micro fleece blanket is the way to go. The sleeping bag in top quilt mode should work fine as temps drop. Use it really loose and vented in warmer weather and start tucking and bundling as lows get in the 50s and 40s. With the Jarbidge and that sleeping bag, I wouldn't be afraid to try to go down to freezing or even upper 20s. Wearing a good wool cap or balaclava when it's cold makes a world of difference and can stretch the capabilities of your quilts.
Cheers.
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