I so could have written your post- in fact, I did write a very similar post a month or two ago (called 'getting past the sleep cycle'- it should be around still if you want to look at the advice there).

I spent SOOOOOO long in my son's nursery trying everything, and have now kind of abandoned the idea in favour of two short sleeps (30-45 mins) and one long sleep. When he wakes up in the middle of his long sleep, I try all the usual stuff and it is often a struggle to get him back to sleep. Sometimes it doesnt work, but eventually, usually, it does. The other two naps, I just let him get up after one sleep cycle. This seems to be enough sleep for him during the day, and it certainly saves my sanity to only go through the whole putting him back to sleep thing once a day rather than 3 times. Maybe try this and see if this is enough sleep for your LO. Some kids just need less sleep! My bubs sleeps great at night as well, and from what I've heard around the place, usually at this age they either sleep good at night or during the day but not both!

Good luck and if you find any magic solutions, let me know!!!