Is it difficult to get np5 welfare servant + it's ascension mats?

It's not hard as long as you know how to properly farm the event's currency and play regularly. Luckily, this pages uploads walkthroughs for every event.

It would depend time you have to play. If you play at the start of the event and use your AP before they are fully recharged, you will get to NP5 with no problem. Only use apples for farming or lottery or if you started the event late and you need to catch up.

Judging by my experience, it's only impossible if you're completely new, like, a week old account or something. If not, you will most likely succeed in getting np5 welfare.

I think it's definitely possible to get, although might have to spend a few apples for the NP5.

I speak from experience as a fellow F2P player if I say it is possible. I started just before the Onigashima event, and I now have a NP5 Kintoki Rider + mats. Granted, you have to get some high-level friends to do it. I got lucky they accepted my random friend request. The event also happened during a period where I had a lot more free time then usual, which helped.