The project manager left 18 months ago because maternity and hasn't
been replaced I'm one of the firmware people
As Joe Strazzere pointed out in the comments, this may give you a hint why your project isn't managed properly and your deadlines are slipping. 18 months of basically "free-for-all, do what you want" is going to do that to any project.
deadlines were set by senior people 12 months ago, but they've been
Under what circumstances were those deadlines set? Did they say "one mid level guy should complete this in X time" or "this should take 12 months, given a team of X people working under Y methodology, using Z tools...". You may find that management (excluding project managers, well, good ones) will only take "it will take 12 months" from both of those statements and then wonder why work isn't getting done as fast as it should in their minds.
Should he just hire more engineers and split test them on the same
Throwing more bodies at a problem only works short-term, if the issue is simply "too much work". If there are obstacles a regular engineer won't be able to remove, like your contractors slipping the deliveries, bad task prioritization, release plan changed every day by management etc, you will just run into roadblocks at an increased speed.
Should we hire someone even MORE senior to measure progress?
Why would you waste some mega-senior talent managing projects? That's project manager's job.
Is there a more efficient way?
I don't know how long maternity leave is in your country (here it's 18 months) - is the PM coming back? If not, hire a replacement. Do note, if it's really as bad as it sounds (18 monnths of no supervising from PM sounds like it), you may not like what the PM has to tell you.
Work with him to release your product in stages (core functionality, nice to haves, strategic client integration etc. ) and not all at once.
Stop having a bus factor of 1 - both for your PM and your sole mobile dev. What would you do if your iOS dev quit today? Even if you somehow hire his replacement on the next day (hint: you won't) you are looking at even more delays while he tries to figure out what he's supposed to do.
How is the CEO (the decision maker) supposed to know whether the engineers are doing their jobs too slowly, slacking off, or whether the work is genuinely difficult?
He's supposed to hire people who can manage those things for him and not let things fester for 18 months.