It sounds like you need to wait for your 10/10 paycheck to be issued.
Firstly, if the employer's share of the combined premium is $542.70 pp but the imputed amount is $273.60, then it sounds like your solo coverage costs them $269.10. There's no reason the costs have to be exactly equal. Maybe the insurers charge an extra $9 per month to administer DP benefits? It's not really important.
The situation with the retroactive premiums is common. Typically, the employer would use September deductions to pay for October premiums (to be at the insurer before October 1). If your DP coverage starts 10/1, then there would be at least two back premiums the company has to pay on your behalf, and maybe 3, depending on how the cycles run. I would expect that on your 10/10 check, your employer will have to pay to the insurer, $542.70 for half the November premium, plus $273.60 x 2 for the October premiums for your DP. Maybe you started coverage on September 1 and the employer is just getting around to adjusting your payroll, so the employer also has to retroactively pay $542.70 for the September premiums.
Assuming that your 10/10 check will have to account for your current total coverage of $542.70 (of which $273.60 is imputed income) plus $1094.40 of retroactive payments to the insurer, then what you should see is not a deduction of $1094.40, but you should see increased tax withholding based on that amount. It's not clear to me if the employer rep is saying you will pay $1094, or you will pay the tax on $1094, and I think you just have to wait. Maybe the rep was simply not precise in their language.
If you pay the entire $1094, then they are not imputing the income to you, they are simply charging you the full cost of the insurance. Maybe they have a policy that says they do this when DP coverage starts? You may need to ask. DP benefits are not regulated by any federal law, although they might be regulated by laws in your state, and maybe it's just their policy to bill you for the full premiums when you first enroll. I think you just need to wait for your 10/10 check to see how you are taxed.
For future checks, you should see your take-home pay go down by the amount of tax that will be withheld on the $273.60 of imputed benefits. That should be 6.2% OASDI, 1.45% Medicare, and maybe 22% for federal and 5-10% for state taxes depending on your state. The regular paycheck after all the retroactive adjustments have settled down will be much more predictable and easy to explain.
