>>39837852"Welfare" in gacha games refers to units/characters/gear/rolls etc. that the game gives to you for free. Typically as part of story progression. In the case of this game, the Welfare Sync Pairs would be the ones you get at the end of each chapter - Brock, Misty, Erika, Flannery, Skyla etc. A "welfare 5*" is therefore a free 5* Sync Pair. Which would be Rosa as she's given to you early on, is 5* and thus has the properties of a 5* (high level cap).
>>39837945Doing 30 one-a-day paid rolls for 3,000 gems would give you a 90% chance of a 5*, so almost certainly, and you would probably get multiple, or possibly none. Conversely the 10 paid roll will guarantee you a 5* no matter what, so I guess it depends on what you want. Do you want a guaranteed 5* or just lots of sync pairs in general?