The reason for the 15,000 character limit is because of the demographic. Readers using Tapas are people on the go and typically only read for short amounts of time, 1 to 3 minutes. The average reader does about 200 WPM and 60% comprehension. The average length of English words is 4.5 letters, or 5 characters. 5*200*3= 3,000 characters. Doing 15,000 characters is a 15 minute read. Most users aren't going to get involved with something that long when they only have 5 minutes in line at Starbucks.
The idea behind serialization is that you sell lots of small inexpensive pieces of the pie and all those small pieces add up over time and before you know it you've sold all the slices at a 30% mark up vs. selling the whole pile at once. At 5k words per episode your entire 80k novel will only have 16 slices. If you want $10 for the novel, you need to get $.63 a slice, or 756 coins per slice/key, making your keys the most expensive on the entire app. Someone is going to see 756 coins and say heck no that's too expensive. Plus if you give 1 or 2 keys away to get people started and hooked to buy more then you are giving away a huge portion of your novel for free and have to charge even more per key to try to reach your $10 goal.
The idea for the short episodes is to get someone to read one or two and then get them hooked and they can't wait for the next update. A long block of text has an overwhelming effect upon the reader. How many novels do you see with only 16 chapters? Chapters are a natural break or resting point for readers. A lot of readers do a chapter a day before bed. If the chapter is too long, readers get exhausted half way through and give up, or even worse, never even start because they don't have half an hour to give up to read a 24 page chapter.
Also if everyone is writing 100 to 200 episode novel bites and you write 16 episode mouthfuls, your novel is going to look like a giraffe standing in a colony of meerkats. Readers are used to the pace of quick bites and a mouthful is going to definitely trip them up.
If you are doing novels for free, there is no compensation other than ad revenue (which is small) and tipping (which varies wildly from one creator to the next). You'd be best off pitching a submission to the acquisitions editor, Gabby Luu, and see if you can make your novel a premium series. Keep the Tapas demographic in mind when making your pitch. Top sellers on the app are romance, fantasy, LGTB, and teen.
Definitely write something formatted for the platform. It's more likely to A) get picked up as a premium series and B) have higher sales.
Good luck.
