1. The ability to use more than one GPU comes at a price premium, so I'd propose to drop it, you can invest the money better elsewhere. If you want more graphics horsepower, you can simply sell the 1070 and get something stronger like a 1080 Ti. I'll still include a second motherboard that supports it this time though.
2. No overclocking is cheaper, however, I'd seriously consider doing at least some light overclocking, espcially with AMD's Ryzen: You can overclock them all without a price premium, so it's practically wasted performance. Just my opinion.
3. That should be doable with most motherboards these days.
4. Same here, you'll have a hard time finding something with less than 4 SATA ports.
5. 7-8 ports on the back panel exists, 6 is pretty much the standard nowadays.
6. Well, that always broadens the choice
So, here comes a list of motherboards that fulfill your requirements (at least the ones I know now):
ASRock: Fatal1ty X370 Gaming K4 (with SLI support)
ASUS: Strix B350-F Gaming (no SLI)
Gigabyte: AB350 Gaming 3 and AX370 Gaming K3 (both no SLI, former "only" 7 USB ports on back panel - all others here have 8)
MSI: B350 Gaming Pro Carbon (no SLI) and X370 SLI Plus (with SLI)
I stopped at the price point of the MSI X370 SLI Plus, as any more expensive boards should feature everything you want. All those boards support CPU overclocking, there were no A320 motherboards that met your requirements.