Questions like this one are sort of like asking how one might go about removing an appendix. Now, it's true, an appendectomy is not the most complicated surgical procedure, and if that's all you want to do, then you certainly don't need a whole lot of other medical training or knowledge to competently perform appendectomies.
The thing is, it might be a good idea to first figure out whether there is anything wrong with your appendix, such that performing an appendectomy would be something you actually need or want to do.
If you are talking about a typo of a generic word being used for purposes for which the generic word is, in fact, generic, then I'd first want to know what it is you believe applying for US trademark registration is going to do for you.
If your question 4 goes to the main point of what you are after - i.e. "if I have a trademark registration can someone else sue me?" - then it's probably the main one worth answering. Of course, someone else can sue you. Trademark owners sue one another all of the time. And, if that happens, then having done odd things in your application, having hired an assembly line operation to mindlessly file it, or having filed an application which was refused on the basis of similarity with a pre-existing mark, are all things which are going to put you in a worse position than if you had not filed in the first place.
The primary, and strong, defense for domain names which are typos of generic words and being used for purposes for which they are generic is that they are typos of generic words and being used for purposes for which they are generic. Filing for trademark registration throws your best defense into the trash, since it is a statement on your part that you don't consider the use to BE generic in the first place.
But just to clean up a few minor points...
US trademark registration applications can be filed on an "intent to use" basis. An intent-to-use application will be examined like any other. If it is eventually deemed allowable, then you will have to demonstrate use of the mark on the specified goods/services before it proceeds to registration. So, no, a mark does not have to be in use in order to apply for registration.
Here's a decision I obtained just today for a client:
https://www.adrforum.com/DomainDecisions/1857174.htm
Complainant is Pocketbook International SA / Batmore Capital Ltd. (“Complainant”), represented by Anna M. Vradenburgh of One LLP, California, USA. Respondent is Domain Admin / SiteTools, Inc. (“Respondent”), represented by John Berryhill, Pennsylvania, USA.
The domain name at issue is <pocketbook.com>, registered with TurnCommerce, Inc. DBA NameBright.com.
...
Respondent has rights in the POCKETBOOK.COM mark through its own trademark registration with the USPTO claiming the services of being a software application service provider (e.g., Reg. No. 4,099,793, registered Feb. 14, 2012).
...
One key factor is that Respondent has rights in the POCKETBOOK.COM mark through its own trademark registration with the USPTO (e.g., Reg. No. 4,099,793, registered Feb. 14, 2012). While the existence of a corresponding trademark registration does not automatically confer rights or legitimate interests on a respondent, it does require a complainant to show why the Panel should disregard a prima facie valid registration of the respondent (for example, where the evidence demonstrates that such trademark registration was obtained primarily to circumvent the application of the UDRP).
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Now, in this case, the Panel did not find that the Respondent's trademark registration "was obtained primarily to circumvent the application of the UDRP" but, quite frankly, the entire trademark issue was an unwelcome distraction in this proceeding. You'll notice under "legitimate rights" that the panel ended up with:
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Respondent claims to have legitimate interests in the <pocketbook.com> domain name. Specifically, Respondent argues to have made demonstrable preparations to use the domain name for a mortgage calculator and financing service. Furthermore, Respondent argues that the disputed domain name is comprised of the term “pocketbook,” which relates to mortgage and financing services. The Panel notes that such use of the disputed domain name for a mortgage and financing services website appeared to commence in 2014 and the domain resolved to a pay-per-click website with links to finance related topics as early as December of 2011. This considerable time period of use supports Respondent's contention that it has legitimate interests in the mark "pocketbook" in relation to a bona fide offering of services.
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In other words, the trademark registration - which was the subject of a huge amount of argument in the case - was really more of an unwelcome distraction to the fact that my client had been legitimately using a dictionary-word domain name for a number of years for purposes relating to the general meaning of that word as a financial-related term.
Honestly, for all the time and effort that silly distraction consumed, it would have been a lot simpler if my client had never run off and obtained a trademark registration in the first place.
But, the important thing to notice is that having a trademark registration is not some sort of "free pass" to win a UDRP. Certainly, there have been cases in which having a trademark registration did not result in an automatic win for the respondent. The most famous of those sorts of cases is <madonna.com> in which it was plainly obvious why someone would highly value that domain name (and was certainly obvious back in the day when Madonna was still producing new work). The domain registrant had obtained a Tunisian trademark registration because, yeah, sure, I guess he was planning on doing business in Tunisia (but, really, Tunisia at that time granted trademark registrations in 24 hours). No one believed that there was any reason for the trademark registration other than as an artifice to attempt to defend a UDRP.