Thanks, the way you have included links to possible laboratories and companies is great!

 Which arguments are keeping the billionaire money from funding it stronger while the metformin trial study did get it's money?

I guess one reason is that billionaires don't have time to do their research in detail, to see for themselves whether de Grey and Sens are worthy, but rely on reputation and opinions of popular names in the field. And the reputation of SENS seems to have suffered a bit initially, when many scientists found their proposals for rejuvenation too bold or futuristic https://en.wikipedia.org/wiki/Aubrey_de_Grey#Criticism

Yes, organisms (even worms) are way more complex than any codebase so far, and researchers don't fully  understand  them and yet - as you can see from this and other reviews - very significant lifespan (and healthspan) increases have been achieved. Even in mice and rats, which, aside from an inferior brain, are about as complex as humans.

To me, this indicates that the aging process is quite malleable, with many ways to tweak it, which researchers are finding through theories, educated guesses and trial and error.

Just of the top of my mind: an example lab that makes (relatively heavy) use of computational approaches is that of J.P. de Magalhaes (U.of Liverpool, so not too far from yourself).

More generally, I believe many labs nowadays could use the help of a software engineer. You could read more on aging research (this article has great links, AFAICT), choose a direction, a lab, and ask them if you can help them. 

I might to the same, one day.

It depends on what you mean by "new resources". In your text, you wrote "One more researcher, or one more research grant will add little to the rate of progress. " -- and that's what I argued against, above.

Simply put, more researchers & companies=> more longevity-influencing factors to be evaluated => higher chance to find ones that work, and work better. 

I've been following the anti-aging field for almost 7 years: research news, overview articles, reviews etc. 
I don't know the author of OP, but I can say that the article he wrote here is as good as it gets (one of the best I've ever read), and, up to date. So are the recommendations he makes for further learning/immersion. I happen to have read many of the articles he cites, and they are all of very good quality.

The names he cites (A. De Grey, Sinclair, Barzilai and others) are stars in this field, mentioned in most other anti-aging blogs as well. 

IMO, the kind of background this author has (not exactly working in aging research, but still within biology and familiar with research in general) is great for writing an overview like this: because it is less biased in terms of a preferred theory and approach for anti-aging - unlike (to various extent) reviews written by a star researcher in the field. Even this author has a preference (SENS) (but perhaps for a good reason, you'll have to judge yourself).

So, the pointers given here are all great (just reddit/r/longevity alone will give you as much immersion and leads as you want).

Personally, I read/follow some of the sources he recommends, and also follow this blogger https://joshmitteldorf.scienceblog.com . 

I think the direction you've chosen, tissue engineering, will be very useful. One important organ for aging to be rejuvenated/replaced is thymus, whose degeneration is a major cause of declining immunity with aging.

"No matter how much you supplement the body, eventually deleterious mutations will accumulate." yes, however, in nature, deleterious mutations don't accumulate in all species. Hydra is an example/exception. It replaces its cells at such a fast rate, that this is an important reason why it doesn't accumulate mutations (and other damage) and manages to be biologically immortal. 
In fact, the direction you've chosen for research would do the same but at the level of tissues/organs, not cells. 

I have to disagree on 2 points: 

1. "When looking at the graph you present, a clear trend emerges: the more complex and larger the organism, the less progress we have made on slowing aging for that organism" -- the trend is not clear. Rather, the worms are an outlier on that graph. Mice are much more complex than flies and killifish (and much closer to humans) and yet, the results achieved are on par. 

2.  "The fact that there are 130 companies working on the problem with only minor laboratory success in the last decade indicates that the marginal returns to new inputs is low. " -- Most of them have their individual approaches to "the problem" . There are orders of magnitude more variables that affect the health and lifespan, and those companies are trying just a few of them. So, they are just scratching the surface of what it needs to be tried and done. More researchers and companies is definitely what is needed.

On "Call to Action" : 
Besides sharing, learning and donation, why don't you encourage research-minded people to actually participate in / join the research? 

First, students or young researchers in position to choose or change their research direction.

Second, others: since the audience that will fully read such an article is likely to have useful research skills (biology, science, computer science etc ). Some may be willing to volunteer with their skills and join some project.

Thanks for correcting me, I didn't know turtles and tortoises are different!
Another example, at least as good, of negligibly senescent tortoise, is Gopherus agassizii  https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4157354/