[1.] Yes. A criminal acquittal doesn’t preclude a civil lawsuit out of the same claims. First, the acquittal resolves only that guilt couldn’t be proved beyond a reasonable doubt (requiring, say, a >90% confidence level); the standard for civil liability is preponderance of the evidence (which requires just >50%, or perhaps ≥50%, if the injury is easily proved and the burden is then shifted to the defendant to prove self-defense).
Second, liability could be based on a negligence theory; the charges against Rittenhouse were based on the theory that he acted recklessly or intentionally, depending on the charge. Of course, the negligence inquiry made its way into the analysis, because the self-defense claim turned on whether he reasonably feared death or serious bodily injury; that would play a role in any civil claim as well. But at least in principle, there might be more room for claims of unreasonable behavior in a civil negligence lawsuit than in a criminal homicide, attempted homicide, or reckless endangerment case. (One way this could happen is that the standard for criminal negligence in criminal cases is generally higher than for civil negligence, though in this case the jury instructions didn’t seem to reflect that.)
This is why, for instance, O.J. Simpson could lose a civil wrongful death lawsuit even though he had been acquitted at a criminal trial. On the other hand, if someone is convicted at a trial, with proof beyond a reasonable doubt, that would generally make him automatically liable in a civil lawsuit based on the same facts and on a similar legal theory: If guilt has been proved at a >90% confidence level, that necessarily means it has been proved at a >50% level as well, but not vice versa.
[2.] What about the money? How much is an 18-year-old likely to have? Well, he did have success in raising funds for his criminal defense, and he might be able to raise funds earmarked for a civil defense as well, but I doubt that anyone would donate money to him just so plaintiffs could take it in a damages lawsuit. (I can’t speak to whether there are any unrestricted contributions left over from the criminal case fundraising.)
On the other hand, if Rittenhouse’s parents own a home and have homeowners’ insurance, that may well cover a wide range of negligence claims against their minor children as well (many homeowners’ insurance policies do that), and not just claims stemming from injuries within the home. It’s possible that a claim that “Rittenhouse was negligent in shooting me, because his fear that I would kill or seriously injure him was unreasonable” would be covered by such a policy, though it of course depends on the exact terms of the policy. Such a policy would cover both the defense costs and a potential verdict, at least up to the policy’s monetary limits; and it could provide money for settling the case (which is of course how many cases are resolved).
This having been said, my sense of the Rittenhouses’ economic circumstances (based on a quick glance at media accounts) is that they’re likely to be renters, and unlikely to have other sources of liability insurance. The main source of such liability coverage is generally homeowners’ insurance, or, for a small and wealthy percentage of the population, umbrella policies or similar liability coverage.
[3.] Naturally, a jury may well conclude that Rittenhouse wasn’t negligent, even under a preponderance of the evidence standard, in which case he’d win the civil lawsuit. (Self-defense is a valid defense in civil cases, though again the standard is preponderance of the evidence.) And, especially if there’s no insurance policy available, potential plaintiffs may conclude that it’s pointless to even try suing. But in principle, civil liability following a criminal acquittal is possible.