Last few mornings I've had 40mins again in the morning. Maybe add on some more morning A?
There is always a chance that something else disturbs sleep for a few days, such as teething pain or a cold coming on but the symptoms not yet showing, if you feel it is not anything else then yes add some more A time. FWIW mine will have been doing more than 4hrs at 10 months as he was always the high end or above the guidance times for his first A of the day - subsequent A times were not as long.
I'm having to walk her in the pram which is APOP as she won't go down in her cot under tired. Do you think this is ok as I don't want her to need the pram to sleep in general?
The good news is that the prop or habit will likely only be linked to this one nap and not every sleep, and the other good news is that you will only need to APOP until that nap is dropped so it isn't for ever it's a shorter term thing and very useful if you can cope with the APing (honestly it can be hard work but for instance mine just needed putting in the car at the right time and we were set for the nap, I did have to drive every day for a few mins to get him off to sleep for the APOP which can feel like a drag if you don't want to go out every day but overall I didn't mind really).
Mine always continued his first 2hr nap at home in his cot totally independent and night sleep remained independent too. From my experience on the boards habits are made per sleep so just don't pram nap the first nap and you will be ok.
Is that common for A to stretch at 9.5months?
I wouldn't say it is common but for sure all LOs are different and some do need a long first A, others need a long last A or they will refuse BT, it's just about adapting to your LO. It is also not unheard of for a LO to drop the second nap and move to 1 nap even at 10 months. Mine was quite early at 11.5-12 months moved to 1 nap but some are even younger. If it turns out your LO is on the way to dropping that second nap early then it's likely better to go with it than fight it. When you move to one nap you will need that long first A, a decent second A and an early BT to avoid OT.
hth