-On Prem, SP 2013, Site Collection that was migrated from 2010

So, I have [read: inherited] a SC with several nested webs. Ever since we've migrated I've had sporadic issues flipping webs to Oslo, either:
*server page errors after selecting the color theme
*(top) navigation goes missing

Would love to throw my hands up and rebuild the whole SC from scratch but that wont come for a few months (due to many custom lists/pages/DVWPs/forms), instead I'm trying to softly switch the ones I can...

So I tweaked the navigation settings from root-sub-sub-sub-sub-web(s) to use the parent nav. On the root SC/Parent, when I flip to Oslo i have no navigation bar up top! I check the root SC Nav settings and there are indeed sites to display under the "current nav" section, additionally if i manually add some they don't show either.

Using designer, tried resetting the Oslo master, page layout and page to site definition; tried toggling the Root Site publishing feature (don't wanna do the Site Collection feature as that will probably break stuff in the webs) Still nadda. If i flip it back to Seattle the Nav reappears and displays the global nav stuff.

Lastly, for clarity[sic]: upon inspecting the page, the div(s) for quicklaunch are there but everything below them is missing, i.e.:
*ct100_quicklaunchnavigationmanager
*noindex ms-core-listmenu-horizontalbox
...are missing, which obv contain the links on the working sites/pages

PS: Sorry I know it's obscure, cont pull screen shots off the domain and yes I'm ignoring the seemingly stranger "server error using change look" stuff on some webs for now.

TL;DR: my navigation bar doesn't appear when using Oslo.

Thoughts?

EDIT: OK, I figured out my issue, hopefully it helps folks as a gotcha, despite how dumb i feel about it now: -The Oslo MP actually does a few things (skipping the content zone width and security trimming of random thing explanation) one of which is change the navigation controls:
--the legacy top [global] nav field is moved to the bottom of the page and hidden
--the quick launch [current] navigation is re-imagined and stuck at the top
~this means when you use the navigation menu (publishing on) the "Current Navigation" controls your top nav (bucks 2010/Seattle Master Page logic etc.)
---YOU MUST ENABLE THE QUICK LAUNCH ON THE "LOOK AND FEEL" SETTINGS MENU FOR IT TO SHOW

...that last part is the gotcha i swear i check 5 times on hunches when i was initially pulling my hair out... afk eating my hat