I can't for the life of me understand why you think I'm saying they planned the DLC exactly. Theres a huge difference between planning the DLC in full detail, and planning FOR DLC. Planning for DLC in advance allows you to develop the game in such a manner that the additional content can be added with less issue.
Here's an example of what I think might have gone down. Start of project, plan to bring every character back, as close to every stage as possible, a selection of newcomers, modes, stages etc. as the project goes on some details fill out, the Pokemon rep is chosen, composers choose the songs they want to cover, movesets change or evolve. They plan to do DLC but don't decide on which characters to put. As the project moves on and they start to think about the DLC in more detail, negotiations start with other developers. During this process ARMS releases and the team decide to include it in Smash with SOME representation at launch and a character post launch when they have more development time. They don't say when, could be season 1, could be season 14 as development isn't going to start on them right away anway, it's just going to happen eventually. Content does not need to be planned in release order.
The point is plans are flexible, they're required to help organise development resources during the whole project and prevent the team wasting effort of developing content that won't be used.
As for why include Spring Man as an assist trophy, seems like the most logical pick, he's on the box afterall. Maybe they knew who they wanted as soon as they saw ARMS (long before us) or maybe they wanted to leave it to a fan vote and just hoped he wouldn't win. I don't know why. I suppose Springman's simple gameplay that focuses on the core ARMs experience means that other characters when developed for Smash would feel like Springman+ whilst the Assist Trophy simple gameplay suits him better. Like I said I don't know, I can't know, but there's a ton of possible reasons and most of them aren't that exciting.
Edit: Also just to add, Sakurai didn't disconfirm Springman, that was the communities decision to interpret Assist Trophies that way. Personally I find AT promotions very unlikely but not impossible.