Is she waking happy from that first nap? If so I'd be tempted to put her down 10-15 minutes later and see what happens. If her nap shortens you know it's OT, if it lengthens then UT? At least then you know where you are. I was always relieved in a way when L did a clear OT nap cos at least I knew where I was then! We don't get many obvious ones from J cos he's lsn too but we get a LOT of 35-40 minute naps. Like I say, not really figured them out yet but an A increase often sorts them for us. I'm in a similar position to you in that I'm on the verge of giving up trying to lengthen our morning nap cos we're getting a good pm nap. FWIW, my 7.5 month old is currently doing a morning A of 3 hrs 45, then a 35-40 minute nap, then an A of 3 hrs 15, then a 1.5 hr nap, then 2 hrs 45-3 hrs til bed.
Anyway, that's what I'd do but I'm obviously no expert seeing as I'm having similar problems so would welcome thoughts from others too!