The law considers all of your traditional IRA accounts as one for tax purposes, so whether the funds are all in one account or two makes no difference whatsoever.
Fidelity is correct that you may not be able to roll after-tax funds into a future 401(k).
The back-door Roth depends on you not having any existing pre-tax traditional IRA funds. If you already have pre-tax IRA funds, you'll end up paying tax on a large percentage of any Roth conversion. The way around this is to first eliminate your pre-tax IRA balance by rolling it into an employer plan before you make your after-tax contribution, but that's not an option for you right now because you are not working.
Since you don't know when or if you might work again, I would advise you to just keep making whatever contributions you can, forget about the backdoor Roth option, and start on a Roth pipeline when you get closer to retirement. The fact that part of your IRA is post-tax will help here because you'll pay tax on less than 100% of the amount converted.