Malthus pointed out an obvious fact that had escaped the notice of 18th century utopian theorists: that if you raise standards of living to any comfortable level, and people continue to marry and have sex, then population will begin to double very rapidly until it outstrips available resources, causing standards of living to fall again until the point where hunger limits family size once more. So to explain how you will raise standards of living, you need to explain (i) where will the capital to raise agricultural production come from? (ii) or, if agricultural production will not go up, will people not have sex? (iii) Or, if people do have sex, will they limit themselves to non-procreative sex? (iv) Or, if people do have procreative sex, will they wait until later in life to start?

These were simply issues no one had ever thought too carefully about. The logic of exponential growth is pretty straightforward -- even in the US, our population could not double quickly without lowering our quality of life (the latest doubling has taken sixty years, during which time our capital stock has sextupled from about $7 trillion to $41 trillion, or a net tripling of physical capital per capita) and if it were to double forever, eventually there would be stresses on the ecological carrying capacity. The solution that Malthus proposed was pretty simple -- people would just wait until later in life to marry and start having sex. His main target, Godwin, essentially conceded all of Malthus' points (although Godwin seems not to have realized, at least in his first response, that children don't naturally suffer 50% mortality before adulthood, but rather that malnutrition increases the overall rate of infectious disease) but instead argued that, after an sufficient level of educational and cultural improvement, people would lose interest in sex. This was the fiercest opponent of Malthus, and his anti-Malthusian solution was that you beat the sex drive out of people. So you can see that this was a debate that needed to happen, so that people could actually think through these issues.

We should also be clear that Malthus stated unequivocally what most people now think is obvious: most young women aren't particularly anxious to have the cares of raising a large family, and that if it were their own choice (instead of their families' or their husbands'), they wouldn't want to start having children until 28 or 30.

Economically, Malthus believed essentially what you believe -- that the most important thing is to increase the amount of capital being used in agriculture in England. His most famous legal endorsement was of the "Corn Laws", the restrictions on grain imports that made food expensive in England. Malthus' thinking was that in the long run cheap food would only increase the population and drive up the price of food again, and it would be better to prop up the price of food to divert investment into agriculture, and more importantly the British food supply would be self-supporting and stable. The goal of this was to prevent what you seem to be calling a Malthusian crisis, that is, a very serious war, famine, or plague when a region already at the carrying capacity sees its food supply interrupted by an unexpected event. The Corn Laws were dubious, but the principle is still advocated today -- many development economists believe that the biggest obstacle to agricultural development in Africa is the constant flow of cheap American grains to third-world countries. Agriculture sectors have very limited prospects for development if they can't successfully compete with imported food; thus one fashionable solution is tariffs on agricultural imports to give native farmers a captive market, driving up revenues and encouraging agricultural investment. This is textbook Malthusian economic policy.

Of course, the world moved forward, and so did Malthus' ideas. In 1798 there was very little known about birth control. By 1823, the leading second-generation Malthusian, J.S. Mill, whose 1846 textbook defined "Classical Economics" for a century and was, in effect, the final statement of the Malthusian doctrine, was arrested for distributing brochures on birth control. In the same decade he was publishing articles explaining that Irish tenants needed to be given freehold over the land they farmed, which he explained again in 1846 in his textbook and yet again two decades later during the revolt. You can look into his career as an MP, as well, if you want to see what the core of radical Malthusianism was in the 19th century. My main point is that there's no real reason to connect Malthus with famine. Malthus was analyzing the logical connections between the food supply and the decisions humans make that lead the population to increase. The purpose was to head off population growth before famine became a risk. Malthus' own idea was to delay marriage; once birth control became a realistic and reliable alternative, that was what his followers settled on. The "Malthusian Society" was founded in the UK in the 1870s (its first goal being to agitate against the laws on distributing information about birth control, still in effect fifty years after Mill's arrest) and forty years later Margaret Sanger founded the organization we now call Planned Parenthood in the US.

Meanwhile the political decisions that were made during the Irish famine were not made by Malthus or Mill or any other Malthusian or, for that matter, any economist. The Whigs came to power with Russell as their PM after the previous Tory PM was abandoned by his party for repealing the Corn Laws with Whig support. The great Whig projects of this period (mostly passed during Russell's previous government) were ending the slave trade, emancipation of Catholics, repealing the Corn Laws, and the Reform Act, which increased representation for Ireland. It's very difficult to look at all this and think either that the Whigs were anti-Irish or that they were opposed to the importation of food. Russell's 1840s government was, more than anything else, incompetent and divided.

As for Malthus' theories - they haven't been discredited. They would barely even be worth calling theories, because they're just trivial analyses of relationships between the growth rate of the population and the growth rate in per capita factor endowments, except that they continue to be impossible for people to grasp. They're the backbone of all population studies and macroeconomics. The more we learn about ecosystems, the more we appreciate his ideas.