Users vs Light Agents

Hi, I've only recently purchased a few Professional Agent licences for my team, however, I'm struggling to see how best to create users/agents for certain use-cases

Use Case 1 - Full Agent (access all Accounts), no problems other than cost
Use Case 2- Client user (we have over 100 clients, each of which require several users), I'm currently setting each client up as an account and pointing them at help centre as end users, seems to work okay
Use Case 3- Internal users who do not require full Agent cost/functionality, some of these users manage multiple accounts, which seems to rule out setting them up as End Users linked to an Account, so I tried creating them as Light Agents, but then they don't even seem to be able to submit tickets (plus can see ALL accounts, not just ones relevant to them). This is currently a major showstopper for our intended use of the system