2504 (or any legacy WLC) itself having MC/MA & we cannot separate that role on it. You can seperate MC & MA functionality in Converged Access product platforms (3850/3650) only.
Therefore here are the answer to your queries
1. You can have a 3850 with MC/MA functionality. But your 2504 will be a separate controller on your mobility domain. You can allow roaming between these two systems configuring them as mobility peers. You cannot register 3850 connected APs to any other controller other than 3850 WLC itself.So you cannot use these two different system as high availability for AP.
You need to have min 7.6.x on your 2504 in order to configure mobility between 2504 & Converged Access system.
Here is a reference post how you configure roaming between 5508 & Converged Access MC (5760/3850/3650)
2. None of the documents listed the 2504 could act as MC for 3850/3650(MA). So my understanding is you should have 5760/5508/WiSM2/8500 as MC. But you can test it & see whether 2504 could act as MC which I doubted.
My understanding is which AP directly connected C3850 WLC another WLC 2504 cannot see that APs because it cannot know information from C3850 WLC. So I should separate in 2 groups of AP that connected to C3850 and WLC 2504 but in the same domian.
The best I think disable wireless feature on C3850 for L2 roaming on WLC 2504 is better, right?
Yes, best at this moment is leaving 3850 as a L2 switch without enabling wireless function. In this way 3850 is another switch on your network & all AP go & register to your 2504. That's simplest solution.
Down the track once you have proper plan to implement this converged access solution you can have dedicated MC (if your need a scalalbe solution). At that time you can enable MA functionality of your 3850 & ensure AP are directly connect to these 3850 stacks (no intermediary switches)
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