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A wrench for those manifold nuts.

In that location the air vent is a 1/8" HyVent. and can be replaced with any of the generic air vents. Taco, Maid-o-mist, Caleffi 502610A. and Honeywell Braukman even has them. It is a pretty standard Supply House part that is often replaced on a regular basis. I would replace at the same time as the tank service because both require draining off the pressure. Maybe not the entire system, but you need to have no pressure on the boiler when you swap them out. I have done both those jobs "On The Fly" (expansion tank, relief valve, air vent and other items) by just getting the boiler pressure to zero and having the replacement part ready with pipe dope and a quick hand. Minimum water loss.

As far as your expansion tank is concerned, there is a valve to isolate the boiler from the tank so that makes it easier. But you should make sure the tank is empty before you attempt to remove it.

A gas conversion burner on boilers that are designed to burn oil makes sense because the oil boiler is designed to use a flame in the combustion chamber and the new gas flame is located in the same place.

A gas conversion burner in a hand fired boiler has always been and will always be a less than ideal design. Since the 1940s the idea of putting a oil or gas burner in a hand fired boiler has yielded a less efficient heating system when compared to a boiler that is designed for a oil burner or a gas burner from the initial drawing board design.

That said, If there is room for both appliances in the boiler room, adding a CI boiler with a couple of isolation valves might be a good idea. The lower priced boilers will be at the 80% efficiency end, while those with power assisted venting will get closer to 85% efficiency. Personally, I would go for the less complicated models and sacrifice 2 to 3% efficiency. The savings on less maintenance and repair will offset the slightly higher gas bill. That is just me.

If I could get a standing pilot without a vent damper boiler, I would go for that. With a millivolt gas valve, you could get gravity heat when the electric goes out. Those were the days!

As far as what size you should get? That depends on how much heat your building requires on a design temperature day. (what you need on the coldest day of the year). You need a load calculation (Heat Loss Calc) to determine that. If you get a 85,000 BTU system and you only need 45,000 BTU then you are going to waste $$ on the bigger boiler and waste $$ on the operating cost. You want to get close to the correct size.

But as did happen in the 1940s when older coal customers kept their homes at 80° to 85°F with the coal, when the oil burner with a thermostat was set at 70°, there were many complaints from customers. Setting the thermostat at 70° was what the instructions said. but the home felt too cold and the customers that were afraid of that new fangled flame in the basement were afraid to set the thermostat at 80° in fear of blowing up the house.

When you banked the fire on your boiler for the overnight, did the temperature get warmer before it got cooler? And that cold room in the morning reminded you to stoke up those flames in the boiler every morning.

The automatic heat may take a little getting used to.

What I said is the couple of extra elbows and length in the OP's PS loop is not an issue. I would not call what he has an install error.

They did a pretty decent job with the DHW pipes in the way.

Yes that bracket simply holds the expansion tank in its proper position. For years we installed them upside down (and it works), yet the instructions say it needs to be in the traditional orientation- so be it. (From a practical laymen’s standpoint- the bladder inside the tank—- I fail to see how it matter because “pressure” doesn’t know direction)

That’s a 3 port, so we can pipe the plumbing/water from the ceiling into that “plugged” port. But this was a quick swap: I would have used a 2 port here but I simply have extra 3 ports in my inventory—-I need to use them up. (I used up all my 2 ports a while back)