To answer your first question, yes I have gone back to an external. Well it's my first time actually, since I only got into backpacking a few years ago.
I picked up 2 Jansport scouts on Craigslist for $30 total. After taking the top two bars off, it ways 3lbs. The same as my internal weighs and it cost me over $150!
My first trip with the external was last weekend, and it was great! I don't think I'll be going back for quite some time. It had a bit more volume than I need, so I'll be playing with some alternative ways to attach my gear.
You mentioned needing to take a separate day pack, why not bring a slightly larger day pack and use that as your main pack body? I think this is what I'm going to try next trip. I have a packable pack that just might work.
With a total day 1 TH weight of about 25lbs. There is no way I would be comfortable with a frameless at this point.