Hello, welcome to BW forums
I agree with FPT about increasing the A times during the day and moving to 2 naps. it can be a bit messy during the transition as on 2 naps you might need to put down early for BT whilst on 3 naps, with the longer A time, you might find BT being really very late. It is a transitional period though so if you keep your goal in sight and keep moving on you will get there.
Although you say he never slept a long stretch at night I do think the current difficulties sound like he is UT at night (under tired) and the refusal at nap time would also indicate this.
For now I would aim for:
WU 6
A 3hr 15
S 9.15 - 10.15 (or hopefully longer)
A 2hr 30 (move directly to 2hr 30 day 1, then increase to 2hr 45, then 3hr)
S 12.45 - 2.15
A 2hr 45
S 5.00 - 5.30 CN
A 2hr 30
BT 8 (temporary late BT until CN is dropped)
and if there is resistance at the 5pm CN I would abandon the attempt and move directly to BT routine aiming to have him asleep by 6pm. As the days go on you will hopefully see the two naps of a good length and no need for the CN, BT moves earlier. I would suggest you aim to be transitioned to 2 naps within a week to 10 days and not allow this transition period to drag out.
Here is a guidance routine to aim for:
WU 6
A 3hr 15
S 9.15 - 10.45
A 3hr 15
S 2 - 3.30
A 3hr
BT 6.30 (if any nap either shorter than 1.5hr then bring BT earlier)
So if I have read correctly from the book at his age when he wakes, if he is crying I would go in, offer to pick him up, cradle him horizontally and NOT wait for him to stop crying but put him down whilst he is crying? And then repeat until he (hopefully) stops??
Sounds like you have this right. You can follow this method to the letter and be confident that by your being with your LO the stress levels are low. Crying might actually be louder and last longer as LO protests to the new expectations but he is not abandoned and he knows he is safe. It is not a no-cry method.
You can keep a hand on him or rub or pat between the picking up so that he knows you are there, this additional comfort can be reduced and weaned once he is falling to sleep in the cot better.
Like FTP I do not have great experience in PUPD as my LO was self settling from a younger age, however I did use an adaptation of this method when I moved my LO from his baby hammock into his cot-bed and you might want to think about your personality, your LOs personality and how you initially taught him to self settle for naps and make some adaptations yourself. My experience was that my LO was used to self settling and even found it hard to fall to sleep with me in the room but during this transition to the cot-bed he was very unsettled and crying. 3 nights of poor sleep. Mine was 9-10 months but I didn't put him down crying as per the method, I picked up and fully soothed in arms then put him down when calm and sleepy.
Each time I put him down he'd cry and scream again.
He eventually got to the point where he was SO tired that when I picked him up he fell to sleep almost instantly, like inside 30 seconds, by the time I had lifted him up to my body he was nodding off again but the second I put him down screaming. I then positioned myself so that I was leaning into the cot, picked up but kept him horizontal and kept him inside the cot, a few cm from the mattress. He nodded almost instantly again. After a few times of this, him nodding off the second I picked him up, I put my hands on him and took his weight without lifting off the mattress. Next step put hands on him in the same position as if lifting but did not lift. He fell to sleep on his mattress.
When I say to consider your own personality etc, it is because PUPD is not a method to be taken lightly. It is hard work, it can be very noisy and test your patience and it is a method you need to commit to. If you give up half way through it doesn't teach LO anything and all the crying (and your won upset) is for nothing.
You might prefer to hold off a week whilst you get your routine in place and then look at how the self settling for naps is going and see what is happening with these NWs before you start the PUPD.
Sorry for so much info in one go. Hopefully this will help some.
Edited to add: PUPD is not an appropriate method to use whilst there is a paci prop. You can use PUPD and drop the paci at the same time but wouldn't be able to use PUPD and continue with the paci.
The other option is to continue with the paci but to use shush/pat to sleep train. Tracy didn't recommend LOs fall asleep with the paci to avoid it becoming a prop but rather to let LO use the paci for the sucking reflex and then to remove before sleep.
Thanks to Ali for raising my attention to this