So is all grace given by God "irresistible" in this way?
No, it's not.
That even though it is never rejected, the recipient freely accepts it?
If you think through the Grudem citation, it'd be tough to ask this question. The model is not acceptance/rejection, the model is death brought back to life; re-creation.
Can what's made, resist its being made? 2 Cor 5:17
Also, is Wayne Grudem an authority for Calvinism? What weight should I give this definition?
Wayne Grudem is a modern Reformed theologian. He's Reformed, so another Reformed theologian would need to "place" him. He's also well-known among theologians, maybe not as popular as Sproul, but on the same level in theological circles.
Any definition that isn't straight from Calvin, we normally talk about it as "Calvinistic", or best, "Reformed". John Calvin himself was one of a number of Reformed theologians -- again, the most popular in his group. To be "Calvinism" proper, you'd need to find it characterized either in Calvin or one of the Reformed council documents (Council of Dordt, Belgic Confession, Heidelberg Confession, Westminster Confession).
One statement of the Canons of Dordt on this matter is as follows:
And this is the regeneration, the new creation, the raising from the dead, and the making alive so clearly proclaimed in the Scriptures, which God works in us without our help. But this certainly does not happen only by outward teaching, by moral persuasion, or by such a way of working that, after God has done his work, it remains in man's power whether or not to be reborn or converted. Rather, it is an entirely supernatural work, one that is at the same time most powerful and most pleasing, a marvelous, hidden, and inexpressible work, which is not lesser than or inferior in power to that of creation or of raising the dead, as Scripture (inspired by the author of this work) teaches. As a result, all those in whose hearts God works in this marvelous way are certainly, unfailingly, and effectively reborn and do actually believe. And then the will, now renewed, is not only activated and motivated by God but in being activated by God is also itself active. For this reason, man himself, by that grace which he has received, is also rightly said to believe and to repent.