The power pads were used in the older wood rackets to soften up the string bed and provide a 'woofer'
effect. There were also some more modern players that used them like Sampras in his rackets. They are not necessary.
The string inside the knot are used to hold the knot from going through the grommet but my question would be why. If you use a thin string and your grommets are worn badly maybe you need to replace the grommets. If they have been used in the past they tend to wear out the grommets even worse than a good knot so replacing the grommets may not be a bad idea. If you've never done it before it may not be a good idea either. LOL it really is not difficult but many people have troubles the first time or two.