Wow, he's 8.5 mo now? Then 3 h would be on the short side! LOL! Now he's probably ready for more like 3h15 or 3h30. So try those times -- but remember, stay at each one for a couple of days before deciding it's not working. At his age? I might go straight to 3h30 and see what happens there for 2 days, but then if it backfires back up to 3h15.
Yes, on a day where he short naps all day add in a short cat nap. Timing kind of doesn't matter *too* much b/c you really don't need him to sleep more than 20-30 min -- it's just to help him catch his breath before bedtime. If he woke from his nap at 2:15, I'd probably try for a cat nap around 4:45 so that he's up by 5:15 at the latest (wake him if you have to) and then in bed by about 8:15. You want to give him enough time so he's tired, but not so much time as it pushes bedtime too late.
However, if he's doing 3h30 A time, then you probably will do an early bedtime instead:
6 am wake
9:30 nap (1h15, short)
10:45 awake
2:15 nap (short 1h15)
3:30 awake
7:00 bedtime (that's 3h30 later, which is his current A time)
see how that works? Plus, now that he's 8.5 months old we actually don't expect him to take much more than an 1.5 nap. You may see him do one 2 hr and one 1.5 hr, or both might be 1.5. That's fine! Perfect for his age group. So, the fact that he's taking 1h15 tells me you are SUPER close to finding his sweet spot for A times. 3h15 might just do the trick! Or, you can push it a bit and try the 3h30 like I suggested above.
At this age yes, the DF can start to interfere wtih good sleep b/c now they don't need night feeds and they just want to burrow down and sleep, sleep, sleep! Not saying it messes up every kid's sleep, but it has that possibility, so best to simply drop it. You can do it slowly or quickly, depending on if you think he's really used to it. If he's taking a good 6-8 oz at the dream feed, then slowly offer 1 oz less over the next few night. Then he'll make up those lost calories during the day. If he's only taking 2-4 oz, then I might reduce by half the first night and then offer none the next night. That little he's not doing it b/c he's super hungry, just cuz it's offered (Hey, if you bring me a recess peanut butter cup in themiddle of the night, I'll eat it whether i'm hungry or not!
).
Does that help??
And don't worry about your kid getting socialized. We started a playgroup when DS was about 15 months old. That was the perfect time because he was walking and curious. I did it more for me b/c he was driving me nuts going up and down our 3 staircases all day! LOL! And he didn't 'play' with those kids, but he did have to start to learn how to share toys etc. At that point he was mostly on 1 nap so it was much easier to fit in a playgroup. When he was 2.5 we started a music class. That taught him how to take turns etc. And then at 3.5 he started preschool! And he's done fantastic! We didn't do a lot of extracurricular activities, and he's taken to it like a duck to water. So no worries, love!