Calvinists interpret these passages as both pronouncements against the sinful actions of the Pharisees as well as warnings to those who might follow them. But ultimately these actions cannot circumvent God's will: they will be judged for their attempts and sinful motivations, but God's irresistible grace, when offered, always overcomes resistance.

Verse 13

Calvin holds, based on Matthew 16:19, that the "keys of the kingdom of heaven are given to pastors" like these men, and that therefore they deserve strong reproach, because,

though they were the guardians of the Law of God, they deprived the people of the true understanding of it.

Instead of acting as they should—as "porters," assisting people through the gates of the kingdom—they shut the gates and take away the key (Luke 11:52).

Jamieson, Fausset, and Brown summarize it this way:

A right knowledge of God's revealed word is eternal life, as our Lord says; but this they took away from the people, substituting for it their wretched traditions.

Verse 15

Calvin describes the situation in verse 15 as follows:

The scribes had also acquired celebrity by their zeal in laboring to bring over to the Jewish religion the strangers and uncircumcised. [...] Christ declares, on the contrary, that so far is this zeal from deserving applause, that they more and more provoke the vengeance of God, because they bring under heavier condemnation those who devote themselves to their sect.

Calvin thus interprets make him in the sense of bringing him.

Doctrinal implications

The Bible is full of stories of people attempting to resist God's will, just like these Pharisees and those who followed them. But, as John Frame says, "the point of the doctrine [of irresistable grace] is that their resistance does not succeed against the Lord. When God intends to bring someone to faith in Christ, he cannot fail." If anyone is snared by the false teachers, we know that thus far God's irresistible grace has not been extended to him. Calvinists thus do not see passages like this as challenges to the doctrine.

Instead, the passage serves as a clear condemnation of false teachers (cf. James 3:1), and in particular a warning to those drawn to them, as Calvin says:

The [Jewish people] were at that time deeply imbued with false doctrine, and had even imbibed from their earliest years many superstitions. While it was hard and difficult in itself to bring them back to the right path, the chief obstacle lay in the foolish opinion which they had formed about the false teachers, whom they regarded as the lawful prelates of the Church, the rulers of divine worship, and the pillars of religion. [...] It is not therefore for the purpose of cursing the scribes that Christ pronounces against them the dreadful vengeance of God, but to withdraw others from their impostures.

• Calvin, Harmony of the Gospels, Volume 3
• Frame, Systematic Theology, page 145
• Jamieson, Fausset, and Brown, Commentary

The concept of Irresistible grace is summed up this way:

Although the general outward call of the gospel can be, and often is, rejected, the special inward call of the Spirit never fails to result in the conversion of those to whom it is made. This special call is not made to all sinners but is issued to the elect only! The Spirit is in no way dependent upon their help or cooperation for success in His work of bringing them to Christ. It is for this reason that Calvinists speak of the Spirit's call and of God's grace in saving sinners as being ‘efficacious,' ‘invincible,' or ‘irresistible.' For the grace which the Holy Spirit extends to the elect cannot be thwarted or refused, it never fails to bring them to true faith in Christ! (Steele and Thomas, The Five Points of Calvinism, Defined, Defended, Documented, p 48-49)

The important thing in the passage you quote is that the people are responding not to an inward call, but to an outward one. If they become "twice the child of hell" there could not have been an inward call in the first place.

To answer your addendum, there's no suggestion that the person isn't already a child of hell, they are made worse by the Pharisees. That's the point here. The point of this passage isn't to make a doctrinal statement (so it would be kind of dangerous to infer doctrine from it), but it's to point out the pharisees that their attempts at proselytization are so horrible and backwards they end up pushing their converts further into sin and condemnation than they already were!