Making beer with hopped Malt extract is slightly different to making beer using all-grain method. Some of the terms and actions used for all grain are not really applicable to kits. One can of course do whatever one wishes but one might also ask does doing so add anything to the brew.
If you are using plain malt extract then one can and should use leaf/pellet hops for bittering and one might wish to use steeped grain to adjust the colour and flavour. However with a kit that is not always useful. Some kits really do work very well just as the manufacturer intended others can be tweaked and modified to produce a variety of different tasting brews. But to answer your questions:
1- add the sugar as instructed or the beer will be lacking in alcohol. The kit is used to supply the flavour and the body - the added sugar usually supplies the main part of the alcohol. That's not to say that one can't use more extract instead of sugar - but one has to add one of sugar or malt extract, or end up with "small beer". Sugar is the cheapest option.
2 - a bit of a convoluted question with assumptions, but, in 24 litres one might dissolve, for example, 3 Kg of LME for a "typical" beer wort. 5lb of LME for 5gals is "about right". In this case the manufacturer recommends using the 4lb of LME and adding some sugar. I suppose the instructions recommend about 1lb or even slightly more to the made up wort. Together that would increase the OG of the wort to approximately the recommended OG. The subject of composition of canned malt extract is the subject of much debate. Sugar added to the malt extract is one thing and added malto-dextrin is another. The easiest way around this is to by "pure" malt extract (dry or liquid) then one can confirm the purity of the ingredients.
3- adding sugar (sucrose or dextrose) will increase the OG of the wort. Infact adding any soluble compound will increase the OG of the wort - but adding sugar is more useful because it ferments to make alcohol and carbon dioxide (and water). There are two types of carbohydrate - fermentable and non-fermentable. Sucrose, dextrose, fructose, maltose and so on are all fermentable and will be fermented according to the attenuation rate of the yeast. However some sugars like lactose and some polysacharides like malto-dextrins and starch are not directly fermantable by Saccharomyces. If added to the wort these will not ferment to make alcohol but will remain in solution and at best will add "body" and perhaps some sweetness to the final brew. In the given example I suppose one could add up to 1Kg (2lbs)of sugar to the wort to increase the ABV.
Yeast is only as good as it is active. So brand name or what's in the packet could both work well but the only way to find out is to test it. The simplest way (even with a brand name) is to make up a weak sugar solution in tepid water and add the yeast then wait for (say ) 1 hour at room temp. If the yeast is active it will be foaming on the surface and micro bubbling in the body. If it is not then DON'T ADD IT - go get some yeast that can exhibit activity and make up a new solution. A typical solution is made up in a cleaned bottle (boiling water pasteurised) with one cup of boiled water to dissolve and pasteurise two table spoons of sugar, then that solution is further diluted with cold water to "hand warm" temperature. When the solution is at the correct temp - cast in the yeast and leave for 30 minutes. Shake vigorously every 10 minutes to aerate the solution.
Using white table sugar (sucrose) in the primary fermentation is often said to give the beer a sucrose "tang" - others usually say any such taste is masked by the hops. Using glucose (brewers sugar) is supposed to be almost unnoticeable except for the taste of the extra alcohol produced.
Boiling extract malt to make beer is not necessary. The best way to use malt extract is to put some freshly boiled water in the fermentation bin then stir in the malt extract until dissolved. The process will pasteurise the extract (not that is is usually needed) and the very hot water helps the malt syrup dissolve more easily. Next add any sugars and perhaps some more boiled water to help it all dissolve. Then use cold water (I use cold tap water) to increase the volume and reduce the temperature. If the water can be directed in a jet to aerate and foam the wort then that is good for including some dissolved oxygen for the yeast. Add more hot water if needed. When all is done and at the correct temp and correct volume - pitch the yeast. If one wishes to add more hops the BEST method by far is to boil the hops separately in water for the required time (adding bittering hops and aroma hops at the appropriate times), when the hop boiling is complete the "hop tea" can be strained into the freshly made up wort in the fermentation vessel. In a similar way roast or speciality grains can be steeped in hot water for (say) 30 minutes and the resulting dark, flavoured liquid can be added to the wort like the hops. It is a good idea to add all these ingredient to the boiled water in the Fermentation vessel first. Then dilute with cold water.