Tarp without bivy?

Should be fine. I did a five week walk in the High Sierra with a GoLite Cave 1 tarp and a down bag. I didn't have a bivi bag. In fact most nights I didn't use the tarp but just slept out.

You'll be fine. Even if your down does get a little damp, the sun will come out and you can dry everything. The only issue is mosquitoes in june and july. I use a nettent (homemade serenity clone) for this, but you can survive with a headnet and lots of deet.

I've used my GG SpinnTwinn in hard rain and got a little of rain spray on me…which inspired this solution. Use the hooks from your GG bug bivy to hook your rain jacket to the entrance to block rain spray.

When I'm on a solo trip…

When my wife is with me…

I use a "Two Person" tarp. No bivy needed. Only a head net for really bad bugs, but bugs really don't bother me, and I rarely bring one.

Keep in mind that when it gets windy, the wind will go straight through the netting of the bug bivy. Can lead to some cold nights.

Why even mess with a tarp? I can see taking one in the unlikely chance you have rain at night but a tarp just blocks out the stars. I usually go the other route and use a bivy without a tarp.

I usually bring a tarp, but rarely set it up. I always sleep without any shelter, unless it is raining or snowing. Spending a night in the rain/snow in a bivy is not fun.