Yes, "for good" means the same as "forever." Incidentally, in English, both sentences would actually be questions, since the "May I" construct is used specifically to ask for permission. Basically it means "Am I permitted to call you 'My Darling' for good?" To make it not a question, with the same words you can just swap the order to: "I may call you 'My Darling' for good." In that context, "may" means that this is something that perhaps you might do. It may not be what you mean by that, of course.

It depends on where you're at, honestly. In my part of America (Colorado), the words "grifter" and "con artist/man/woman" are very common, but not "chiseler." But it could be different if you're in, say, England or Australia. As for "deadbeat", the most common use here is for people who are lazy and irresponsible, sort of like you mentioned with the father not paying child support, but it can be used of anyone who doesn't pay what they are supposed to or who doesn't do what they are supposed to. Again, it also depends on the country as to how common the various usages are. If a friend owed me money, I'd be more likely to say, "You still haven't paid me back, you worthless bum" but I know in England "bum" means something else entirely and that expression wouldn't work.

Yes, the meaning is the same.

Every one of those sentences could be said by someone and would be correctly understood. When there is a mixture of singular and plural nouns, it's usually best to provide the article before the noun. Also, clothing is a bit strange because while there are plural words, like jeans, it still refers to a singular object. Also, boots (while plural and technically referencing two things) is commonly viewed as a single thing--a pair of boots. All that to say, clothing isn't your best example to try to figure out the correct rule here. In general, when you have a bunch of items, you can either reference the number before each item, or you can group them by items. Suppose, for example, that you had one bookmark, one ruler, one computer, two pencils, two pens, and five sheets of paper. You could say: "I have a bookmark, ruler, computer, two pencils and pens, and five sheets of paper." The "a" before the word "bookmark" will carry through for each item until you specify the change in the number.

While the sentence is a bit long, as another person already noted, it's actually correct grammatically other than the word "provided". The first part of the sentence is in present tense ("Camp attendees ARE engaged...") so "provided" needs to be "provides" to be in present tense as well. Alternatively, you could change the beginning of the sentence to "Camp attendees WERE engaged in volunteer work" and leave the word "provided" in place. However, if I were writing it, I would probably edit it to say: "Camp attendees are engaged in volunteer work in local communities and at elementary schools in remote areas, providing opportunities for them to closely interact with local residents and achieve mutual benefits." But this is just personal taste. If you correct the tense error, the rest of your sentence is fine.

Yes, that is correct. In fact, it's how nearly every American would say that sentence.