I think I more or less answered your question and maybe you just missed it

Corporations donating money isn't "evil corruption" by default (whether it's speech or not is another matter). The problem is when a corporation (or union, or whatever) donates equal amounts to both major political parties. That's not supporting a political position with money, that's just bribery and hedging your bets.
Government stimulus isn't the same thing at all, assuming there is a reason behind the donation besides attempting to gain favor. For a corporation donating to both major parties, I can't see an obvious reason beyond attempting to get personal benefit no matter who wins. Government stimulus can have other motivations.
I wasn't referring so much to government stimulus so much as pointing out that government would also fund both parties equally, at least initially. AS far as bribery, to some degree it is, but if both parties are funded then mostly it's protection money. If I fund your campaign but also fund your opponent, I'm not buying myself much good will - but perhaps if I give you both money, whichever of you wins won't hurt me and might even actually protect me from time to time. When a corporation gives overwhelmingly to one party, that's probably bribery.
Unions are groups of people joining together to further their cause. Corporations are not. If all 200 possum lovers in the US each donated $200 to a post turtle or if they joined together forming a Possum Union and that union donated $40k to the post turtle, what is the difference? Now, look at the flip side. If Possum Stuffers, Inc. (a corporation with 200 stockholders) donated $40k to a different post turtle, is that the same thing? Do you think all 200 stockholders of a single corporation have the exact same preferences when it comes to D or R?
Um, no. A union exists to protect its workers' interests; forty possum lovers might form a possum wrangler union, but a better example would be them forming a possum PAC. A desire to earn a good wage possum wrangling does not at all correlate to a desire for government-funded abortion, or a desire to have one's Second Amendment rights destroyed or have one's energy costs "necessarily skyrocketed". If forty possum lovers wished to fight for the interests of the possums they love, they would form a Possum Political Action Committee and donate to that.
A corporation is composed of people who voluntarily join that corporation by buying shares. In many states, one has absolutely no choice about joining a union if that workplace is unionized. Even in right-to-work states, many workplaces hire only through the union and one has no choice about joining the union if one wishes to work there. By contrast, one always has an easy choice about owning part of a corporation; just sell that stock and buy another. Both corporations and unions will have some people who do not agree with that entity's political contributions, but with the union, acting on that disagreement means suffering damage to one's career. If for instance I am a conservative steam fitter it's quite possible that every employer of steam fitters in my area is unionized and therefore avoiding donating to Democrats requires that I either give up my skilled trade or move out of the area. If on the other hand I am a liberal UPS stockholder, I can easily sell my UPS stock and purchase Federal Express stock. Trade unions' members for example typically vote roughly 40% Republican, yet their political contributions (including donated labor funded from union funds) as taken from their dues are virtually 100% Democrat. You would, I think, find few equivalents to that in the corporate world, but it's more or less ubiquitous among unions, they donate to democrats and Democrats use the power of government to reward them. Both unions and corporations will likely have a substantial percentage of dissenters who don't like their associated entity funding things they find abhorrent, but in one case avoiding this often involves losing one's livelihood whereas in the other it involves merely finding a more politically aligned and similarly lucrative investment opportunity.
I find this hard to believe. Union guys around here where union and non union can work on some of the same jobs make significantly more than non-union. Sometime 25%+ more.
I have experience only with the trade unions, but in general I'd say you are correct that the unionized worker typically earns more, even after dues, than his non-unionized counterpart. Sometimes the non-union shops pays its best tradesmen more if the job market is strong because he is competing with the perceived benefits of the union, but it almost always pays the mediocre tradesmen less even in a strong job market.
Trade unions also provide a benefit to the owner of their projects. Because the union insists on stringent educational curricula and training periods, and maintains (at least in theory) a relatively fixed ratio of master to journeyman to apprentice, the owner is assured of a minimum acceptable competence that might or might not be present with a non-union contractor. For instance, I ran into an area of Florida where some electrical contractors staffed jobs with one journeyman and twenty to forty illegal alien laborers, giving themselves a huge advantage in the bid market. While the project owner reaps only as much of this bid advantage as the non-union contractor thinks he needs to give up to win the bid, the project owner sees all the down side of having a poorly trained, unqualified work force who cannot read blueprints, much less understand them, rather than the skilled tradesmen he thinks he is getting. (In fairness, I should also say that I've worked with some very good non-union contractors; I'd just say that almost all union contractors are at least competent whereas many non-union contractors are not at all competent.)
Unions have given themselves a very bad name, but trade unions remain a force that adds value as well as protecting and benefiting their workers in my experience.