So, I've played this game a bunch o' times, and Elrindir was always investable - but now, no.
I've been playing with the various unofficial patches all along (no other mods), and still continue to do so, but again, until my most recent playthrough, he was always investable. Searched these forums for info, but got nothing pertinent. Checked the USKP buglists and bugfix lists, but there was no info. Checked the UESP, but nothing was mentioned there either.
Recently playtested my game without the unofficial patches (just to be sure), and it made no difference - still no investment option.
I get that in a game this big, there's always bound to be something wrong - I have never had a "perfect" playthrough (meaning bug/glitch-free), so maybe this is just the "thing" that glitched on me this time, but I figured I'd throw it out there and see if anyone else has experienced this.
Naturally, I'd be receptive to any kind of fix. I couldn't figure out how to add the investment option dialogue to him via console, but a more savvy gamer might know how.
Thanks in advance for any feedback.