How late/outside of scheduled FP window are u allowed before your FP isn’t any good? I’m wondering this because I have a 1145 Yak n Yeti reservation and was able to score a FOP fast pass for 1215-115. Think I’ll make it?
That’s way too close and the venues not near enough to one another. I would try to reschedule one or the other.
To directly answer your question, I don’t think there is an official policy on grace periods, but have used FPP up to 5 or 10 minutes late before. I think my experience is common, but I don’t know that anything is in writing. “Internet Lore” sort of has the opinion that 5 minutes early until 15 minutes late.
Hope that helps.
Animal Kingdom was where I have had the LEAST luck with getting an extension for FP+. I was pretty disappointed in the CM at the Adventurer’s Outpost who would not let us through about 20/25 minutes after our window had expired. I understand it is low capacity, but I had a one year old who had clearly just woken up from sleeping through my FP+ window. I would think you would have zero luck with FOP and the amount of traffic there.
I had luck on Spaceship Earth as well as a couple MK attractions getting in shortly (but more than 15 min) after our window expired. But I also clearly had a small child. So I absolutely think your mileage can vary widely and you need to be super, duper nice to the CM when asking.
I would think you’d have the best luck doing FOP right at 12:15 and then going to Yak n Yeti after. It’s never been super crowded in my experience. I think the Landry’s card will also get you in there without a ADR.
You won’t make it. If I were you I’d eat somewhere else (Satu’li Canteen?) or try for a later ADR at Yak & Yeti. Or, as Melcort10 said, if you’re a Landry’s Select Club member you don’t need an ADR - the cost of the card is credited to your Landry’s account and taken off the cost of your meal the first time you use it, so it’s effectively free. No reason not to get one. 
You can also use it at RFC and T-Rex, as well as numerous other locations outside of the WDW bubble.
Maybe even try to show up to yak and yeti 15 min early? good luck!
How long does a meal there take? Is it a long walk to FOP from Yak /yeti
We were at AK today! I think it might be about a 10 min walk depending on your pace (we are fast walkers). And FOP was AMazing!!! Especially with a FP! There was 2 hour standby in the afternoon. 8am EMH had posted wait times at 1 hour quickly. I am finding TS meals to take 1 to 1 1/2 hours. I would not risk losing that FP.
Also I had researched meals at AK and in the end we cancelled our Tusker House ROL package (we needed to go back and rest- imagine that!) and ended up at Hambre market QS (not on my short list) but it was really great food!