Fair Questions.
In regard to insurance, I would, of course, need to consult my agent to see exactly what is required to cover myself and the owner of the properties I worked at. There would be no work done until I had everything I would need.
In regard to warranty on work I did, no problem on that. While everyone has an occasional call-back, I would always take care of a failed repair at my own expense, assuming it in fact was my error. And, no one would ever be charged $50.00 for me to tell them they needed to buy a new refrigerator or range. This happens, and it again is because of appliance repair companies can't (or won't) drive to your property for free.
As to the certifications on certain items, I simply will not do anything that violates licensing laws for which I am not licensed. Simple as that. While I can trouble shoot about any HVAC problem, I stop short of getting into freon or going into the electrical box. For that, get the professional HVAC person. I am saying why pay a pro to change filters, clean coils, clean drain lines, or do any of the smaller things needed? When your tenant's evaporator coil is frozen up, it will most likely be low of freon. I can do the diagnosis, and preparation, and all the pro has to do is come and add the freon. (And the best part is, I will get the pro in behind me for you, saving you a phone call and worry about it.)
On the plumber: When a plumber gets home from his day, he plays his answer machine and hears the calls he has. He sorts them into the most profitable calls first, the ones where he makes easy money. He does not consider that you are a landlord and have tenants crawling you about a stopped line. If he has a high end faucet to install for a client, and a stopped sewer, he is going to do the faucet and get the easy money first. He will schedule your stopped sewer for late in the day, so he can go from that job to his own shower. (Can't blame him on that). You and I know the stopped sewer is a pain for the complex and tenants. I will be there as soon as one of my manager friends calls me because I know first hand the problem the stopped line is for the complex. I have a sewer machine, so I can probably save you about $75.00 to $100.00 over the plumber, and I can be there "yesterday".
On the money part. I am talking about doing this for my "circle of manager friends" whom I have known over these last ten years. From my management time, I know their problems, and I know how long I have sit on "hold" on a Saturday trying to get a "pro" to come to the property. This is why I have learned over time to do my own maintenance tasks. I have a professional plumber, electrican, and HVAC guy in my pocket via cell phone. They can and sure will talk me though any or everything, or will come and help me if I get into something I need them for.
I am not against plumbers and appliance repair people or anything, but I have a lot of experience with them, and I know their ways. I wound up with about four great guys who I used a lot and these know I will get them business that is worthwhile for them. So, they helped me tremendously when I was managing complexes, and they still will.
As said, I really just need to keep myself occupied a couple more years, and thankfully it WON'T be in management. I love the maintenance side, but over time we came to dislike the management part. I mainly retired because wife and I were sick of that part of it.
Hopefully this post will clarify my OP.