I have an example scenario, and I'm hoping somebody can provide step by step instructions for how to do it efficiently, which I assume involves using instances.

Looking down, a series of platforms that look like [][][][][][][]. You would approach them from either side. As you get within X units of one, it will raise, and if you leave that zone, it lowers. This way, each platform of the "bridge" will automatically raise and lower itself so that you can easily walk across it, giving the illusion of a bridge that is forming itself just for you as you approach.

My current method is to create a VMF with the relevant things in it (panel brush, panel arm, player clip brush, trigger area, logic relays, etc), all of which point to names that include "&i". Before copying and pasting it into my final map, I do a "Replace" and replace "&i" with "&#", incrementing the # with each paste. This works just fine.

But what if I want to change something like, say, the trigger zone width? I'd have to do it manually to each one. My understanding is that with instances, I can just alter the instance VMF and it will change all instances, but I don't understand HOW to do this. Currently, if I just copy and paste the instance repeatedly, the first trigger the player enters will trigger ALL of the platforms, because they all have the same name. How do I differentiate between names?

Similarly, lets say I create a somewhat complicated instance, as in one for a map I'm currently working on. It involves a cube dropper and its associate button, an excursion funnel, another button, and a button to control the excursion funnel. I will have 3+ of these sets in my map. Currently, I am doing my above method of replacing "&i" with a &# before pasting it into my map, but if I need to change something, I have to manually change all of them. How do I use instances where each instance is incrementally named?

You can't change the size of a brush via a variable. The brush size of your trigger_multiple will be the same in every instance you, since that's the way instances work. I don't think you would need to do anything special with the I/O, like you shouldn't need a func_instance_io_proxy or anything.

I would start by making a single panel work the way you want, then make it into an Instance and try placing 2 of them and see how that works.

I now see that I was totally not understanding something related to instances. Indeed, I wasn't even placing them correctly due to a badly formatted post I was reading (not something here).

Thank you for your help!

No problem. Instances are a little weird to work with at first if you are unfamiliar with the concept. You shouldn't try and use them for EVERYTHING since not everything needs to be an instance.