1) Yup, employees can leave. We know that. If have a "key" employee, then need some strong reasons they will stay, and even with those reasons they may leave because of something about their marriage, children, health, etc. If you promised them stock and are 18 months late, then they may get pissed, not trust you, and leave. They may get run over by a truck, etc.
We know that.
That's why the founder should be the main key employee. Or, if have a key employee, then maybe they should have a deputy that can fill in in case of an emergency. That's why there are carefully thought out compensation plans for key employees, e.g., unvested stock options.
Once I was in a little company. Since there was some question about my status, I circulated some resume copies. In that little company, some instances of good work I'd recently done made me essential to our business with one potentially good customer and with our main customer. One of the resume copies got me a better job offer, and I took it. As I submitted my resignation letter, soon late at night I got a phone call from the CEO of the little company: He told me that he would accept my resignation "with prejudice" which didn't really mean anything. He was also drunk at the time.
Part of the job of a CEO is to keep key employees happy. That CEO had been treating me as excess baggage, with the mushroom treatment (keep the employee in the dark and feed them BS) until I became important. Then at one point, in the hall, as apparently an off hand comment, with some resentment and no details, elaboration, or discussion, he said "You are becoming an important person around here". Actually I'd just done some work for a week I'd never told the CEO about, work that thrilled our main customer and in effect got us sole source on a competitive software contract. I didn't yet know how important my work was, and I didn't know that my CEO knew anything about it. But apparently some high up guy at our main customer (the US Navy) called my CEO and had a chat about my work. So, as of the mushroom treatment, not letting an employee know they were doing well, etc., my CEO didn't discuss my exceptional work with me. So, all he did was just make the resentful remark in the hall. He was happy as a clam until he got my resignation letter which suddenly put his future with the two customers at risk. He was CEO of the little company, but that little company was just a subsidiary of a much bigger company; so, no doubt, from losing two big chunks of business, the CEO's job was also at risk. He was a dumb CEO.
2) Yup, the year 2000 bubble and crash hurt a lot of startups. But for a startup, the flow of more equity capital is always a really bad crap shoot that can be affected by anything including the hemlines of skirts of teen girls, sun spots, and a war in central Africa, or a drought in the Amazon valley. A startup just CANNOT depend or count on (count chickens before they hatch) on future equity capital. Similarly for future M&A deals.
Instead, if the customers are still happy and the revenue is still there, then that's about the best can hope for; in bad times, commonly anything more than that can be just a red cherry on top of whipped cream on top of ice cream on top of a waffle. Until the bad times pass, what's real is just the waffle. For the going business, just keep that going, be glad there's no second Great Depression, be glad some one customer, the source of 75% of the revenue, doesn't go bust, etc., keep the revenue going, please the existing customers, try to improve the business in promising, incremental ways, accumulate the after tax earnings, keep watching for better times, and be thankful for what do have.
3) Sure, there are lots of ways some lawyers can look for billings. One way is nuisance law suits. So, sure, maybe start your business as a sole proprietorship. As soon as have any decent revenue look into business insurance to protect you from nuisance law suits and pay a lawyer to set you up as an LLC. Then get to relax for a while. If are small, then filing a nuisance law suit against you is not worth the time, expense, and effort. But if are small and some lawyers do file, then -- IANAL but just thinking out loud about what I'd consider doing -- let them sue the LLC. Shutdown the LLC and restart the business under another name; contact the old customers, etc. and keep going under the new name. Also patch the legal hole the lawyers used to attack you. Also, as soon as have any decent revenue, talk to some lawyers and get some protection against nuisance law suits. E.g., I just saw a disclaimer by a major company saying that they don't accept, look at, or use unsolicited ideas from outside -- right away I kept and indexed a copy of that disclaimer and will be sure to use it when and if appropriate. Generally, need some protection against law suits from unsolicited outside contributions -- IANAL, but likely YouTube has some legal walls against such attacks.
Once are a significant company, do check with more than one high end business law firm and business insurance firm on being protected.
4) Sure, can have spies, worms, saboteurs, agents of competitors or unions, thieves, etc. So, use some standard precautions, e.g., the standard rule in security, "need to know". Maybe have some bonded employees. Look into some security firm checking the security of your organization, say, something like a white hat hacker would check the security of your server farm or other computers. Keep the intellectual property crown jewels nicely locked up. When still small, for the daily incremental backup data, maybe have the CEO take those home and store them in a box in his den; have multiple copies of full backups stored with great safety, off site, etc. Make good use of encryption. Etc. Nothing here is new; we're not the first to consider such things; so, there should be some good advice readily available.