All of what you mention is possible for the production of hydrocarbons, but the amount of energy needed to build long chain or high molecular weight hydrocarbons using these methods becomes economically un-viable. There are processes in hydrocarbon processing (from crude oil) that can take large chains and make them smaller (hydrocracking) or take small chains and make them larger (catalytic reforming), but you're generally not going up or down in big steps. Big steps takes too much energy for the end result so it's best to start with a hydrocarbon precursor that is close to what you want to end up with (e.g. pentane into octane or methane into butane). Scaling this process up to go from individual atoms of hydrogen and carbon would be extremely inefficient and costly. We use crude oil for a reason since it is made up of many different sizes of chains. With the improvement in processing the crude oil precursor kerogen, the hydrocarbon industry has opened up new doors to feedstocks a refinery can run. Much of the new hydrocarbon extraction is based on kerogens (think shale fracking) and there is a lot more of it than we can imagine. Kerogen also replenishes itself quicker than crude oil so we're in no real danger of running out. Big Oil is still a problem though, but that will have to be dealt with the old fashioned way, if you know what I mean.