How convoluted can it be?

No

Start the venting low enough to just go under the beam. Or move the boiler to a better location for venting.

The installation manual will tell you how long you can run your vent and what the equivalent length any fittings are equal to. As well as what can and can't be done venting wise.

Try to keep your intake separate from the exhaust as much as possible. Concentric vents can work but typically the intake ends up pulling in the exhaust air which leaves you with premature component failure and cussing out the manufacturer. Do yourself a favor and make sure you bring in clean combustion air.

Not necessarily. The exhaust is pointed away from the building and is forced out at a high enough velocity that it will not interfere with the intake that is drawing air from as close to the wall as you can get. 

The side by side vent cover does the same thing.  The exhaust gets forced away from the building while the hole next to it is covered so the intake comes from the wall in a similar manner. 

When using just PVC fittings, The exhaust is recommended to point away from the building and the air intake should be equipped with a 90° elbow facing down.  That way the combustion air will be drawn from as close to the wall as you place the elbow, but from a loser point in the space than the exhaust which should be lighter than the outdoor ambient air and rise above the intake.  

As far as a industry standard that all the manufacturers follow, there is:

Direct vent is when the combustion air and the exhaust are are from the same outdoor location and the distance from windows can be as little as 12"

Power vent is where the exhaust only goes outside and the combustion air comes from inside the house. That has a 4 ft minimum distance from windows. and doors measured horizontally below, and and 3 feet measured above and air intake or window

This illustration is for power vent where you get your combustion air from inside the building.

And all of these are based on having enough space from an adjacent building like you may find in a urban area where housing is built within a few feet of the property line and houses are only 3 or 4 feet apart.

Basically you need to use common sense to make sure the exhaust gasses that are outside, stay outside.

I found one building in a condo community where there were 2 water heaters and 2 furnaces that were direct vented.  All within a 3 foot section of an outdoor wall that has landscaping that had grown up in front of the venting.  The architect designed it that way to hide those awful looking pipes.  My customer has a direct vent DHW that the thermocouple interrupt switch was butchered by someone that had no clue.  They determined that the thermocouple interrupt was the reason for the failure and got some wire cutters and solder and rigged the wiring at the gas valve.  “Fixed it good” 

I got there just in time to use my soot vac to remove all the carbon from the flue and burner area.  (It is not called soot on a gas heater.Ask any gas company employee)  I determined that at least one of the four exhaust vents was swapped with the intake pipe.   I was able to get the gas valve to operate properly with a new thermocouple and pilot interrupt switch.  And gave the customer the price for redirecting the venting properly and extending all 4 of the exhaust vents an additional 18” from the wall, and painting them green to blend in with the shrubbery.  The price for the work was discounted 20% if they purchased a service and maintenance agreement.  A month later when the $14.95 payment for the service agreement showed up on the credit card statement, I got a call from the Lawyer son to cancel the auto-pay.  The Lawyer cancelled the needed repairs and that was the end or it.  I guess they are still having flame failure problems with the water heater.  Good thing that the “know it all” lawyer stepped in before Mom got the problems solved.    

Mom might have has a trouble free water heater if he didn't intervene

My tankless has a metal version of this, works fine. (followed the manual!)

those concentric are my least favorite way to vent. Certain wind conditions tend to cause them to recirculate. We have seen a number of field modifications trying to correct that condition.

By far the roof vent is best, air intake a few feet away.

Wall penetration with two pipes, at least the separation the manual shows. Or more.

Intake air from inside is sometimes an option, although they tend to be noisier

The cleaner the air, the better. Dust, pollen, grass clippings, bugs, all need to stay out.

The intake pipe is basically a vacuum cleaner.

Here is one from IG, full of bugs!

In some cases a screen, or even air filter is needed.