Hello!
Let’s start with the apron. You are right. The original apron fits the Britax B-Motion 4 Plus awkwardly. I would use the Pinkie apron, which fits Britax strollers better in general. No apron is needed, though, if you have a good footmuff.
The footmuff - you can’t go too wrong there. The Pinkie is fine as well as the Zopa or the Cotton Moose. Since the super long seat, maybe even a longer footmuff would be a good idea, so it looks visually better. How about the universal Lodger Explorer (but that’s an expensive one, worth to consider the second-hand option), the Emitex, or the Kaiser? From your three, I would probably prefer the Cotton Moose - the Combi type may work for you well because of its length and the zip-on add-on.
About the seat liner - only a long one will look really nice in it, and due to the length of the seat, I would consider asking having it tailor-made. It's something completely different when the seat liner fits the stroller perfectly, so you don’t need to correct it all the time. That would be my first choice... And from those widely available that offer a kinda-right length, I would consider the prolonged Pinkie brand, or the Bugaboo seat liner. Elodie Details is alright too.