My post count is Majestic
Doesn't help answer your question but when you have try running your figures though a brolly comparison calculator like this. There are others out there.
Limited vs. Umbrella - Tax Calculator
'CUK forum personality of 2011' - Winner - Yes really!!!!
Lord of the Vectras
Were you registered for PAYE back then? Are you now?
You could probably call the dividends from then a director's loan and then deduct that from what you pay yourself in this tax year (after PAYE). A little dubious for sure, and you'll have still lost last year's tax allowance.
Special Discretion Required.
Fingers like lightning
I don't think there's anything to stop you registering the company as an employer retrospectively. Issue you may have is hefty late filing penalties, as you'd then presumably be submitting a whole load of RTI submissions significantly late. You might find they dwarf the tax savings from including the salary.
Sounds like one of the suggestions you've received is just to backdate a little bit (ie to Apr 2016). By doing that you could potentially get some relief with no/negligible penalties.
Afraid I don't think there's an easy fix now. Doesn't help you, but this is why it's worth at least speaking to an accountant at the outset, even if you don't formally sign up with one. Most won't mind giving you some basic pointers for free to set you off in the right direction. Or of course plenty of this kind of advice is fairly readily available on the internet if you know what you're looking for.