Yes, you need an onward ticket every time you enter.

Yes, it's needed, since you're starting over as if it's your first visit. After entry you'll get the first 29-day visa waiver extension

Immigration rarely asks me, but the airline asks most of the time

There are 3 ways to get an onward ticket. 1. Buy a real ticket and don't use it. This seems to be what you think is the only option. 2. Buy a fully refundable ticket. This is basically free, minus your time. 3. Buy a paid reservation known as an "onward ticket" (search online and you'll find options) These usually run about $15 and are valid for 48 hours, so you buy one the night before your flight. And technically there's one more option 4. Just go without one. PH immigration usually doesn't ask or care, and they'll obviously know you're extending as a tourist. Just tell them that. The problem is some airlines make a big deal of it, and you might have to buy a ticket at the last minute or, worst case, get fined at boarding. Digital nomads and world travelers who move around a lot tend to do this or use the "onward ticket" option. Pretty much nobody uses a throwaway ticket except some expats who worry too much but never get a long-term visa. Best in my opinion is an "onward ticket" or a fully refundable one. It depends on whether time or money matters more to you, AND whether you're sure you understand the refund policy. $1k in credit on an airline you never fly is an expensive mistake.