You are correct, the number is, indeed, 1461. And while it's kind of about math, it's more about systems of counting and not so much about combining numbers. 1461 (or, rather, the symbolic equivalent of the numerals one-four-six-one) represents the same actual value as 391 but in a different numerical base. 

If there was a sentient alien species that only had 6 fingers instead of 10, or if the number 6 was otherwise significant to them, they might have developed a different system of numbers than ours. In their system, 1 would still be 1, 2 would still be 2, and so on until the number 6. Once it hit 6, it would be represented by their equivalent of one-zero. In their world one-zero means 6, two-zero means twelve, two-three means 15, and so on. This is similar to how binary (base two) works: zero-one means 1, one-zero means 2, one-one means 3, one-zero-zero means 4, and so on. This phase of the puzzle is sort of a nod to my favorite Cyan Worlds games, specifically Riven and Obduction, which famously feature non base ten numeral systems. Thankfully, there are tools online that can help you make the conversions between different bases nice and quickly. Anyways, hope that wasn't too spoilery an explanation or, conversely, too confusing. 

As for the odd beacon in the mansion level, If I remember right, it might be an Easter egg of sorts. You sort of had the right idea by clipping through the world, but it was there under the assumption that the player falls off the edge of the map completely, so you didn't quite fall far enough down. In fact, I think most levels have that same portal, but that one might be the only one that actually has a beacon attached. And to be honest, I no longer remember if that was intentional on my end or I just forgot to obscure it for some reason. You've probably already been to the area that said portal leads to. There are one or two much less obscure ways to get there. 

In any case, it's always really cool for me to get a chance to talk to people actually playing the game. I guess it's one advantage of obscurity! Folks very rarely message me about, but l'm generally thrilled to be able to help or just hear about their experiences!