Inner vs Outer VLANs – Understanding the Differences

Answering mostly so this stops popping up as unanswered: There appears to be no way forward for these switches, indeed.

Judging by your switch, I'd say you don't have the budget for a real ethernet tester like those from Fluke Networks.

Your switch is Layer 2 (L2) only and doesn't support routing, so you'd need an external router (L3) to move packets between the VLANs. I would conjecture that your random network issues were caused by either bad cabling or mismatch on speed/duplex between the switch and your systems.

You can look at using iperf configured on a server, and then have each of your client computers run a test from their respective ports to the server (one at a time), then look for interface error counters being incremented to help isolate any issues or misconfiguration.