It is incumbent on you to make sure nothing goes up in smoke on your boat. Not during installation, and not while your family is sleeping offshore. So you have to discover what happened last time. If the explanation doesn't include something that the installer did wrong, you are being lied to and need to just move on.
There better be a WORKING handheld voltage meter on this jobsite. And when this job is complete Captain Wayne better still have one in the tool locker.
Post here the detailed plan is for connecting the inverter to the existing electrical plant. By the time you and the installer have worked out this detail, you will know it well enough to explain it here. If you can't write this out yourself, you haven't leaned your lesson yet. Before handing over the next inverter, you should know every connection point, the terminal size of that point, on, and on. When you post it here, you will have your own expert review panel. I bet that if you sent an email detailing the proposed installation to both the inverter manufacture's and the boat manufactures technical support department, you might get a response of whether the plan is missing something.
If your installer completes his half of the bargin, does clean work, and documents it well, he will have a happy customer talking about him on the internet. Through this maturation process, he is likely to find himself installing equipment for some other boaters on this site.
On the other hand, if he insists that it is not necessary to install the system as per the manufactures requirements, you need to get him off your boat before his actions lead to tragedy. Although this is simple stuff, the cost of not doing the job right can be huge.