1. You should be able to find an assistant guide (向导兼背夫)at $20 per day or less. The price includes lodging/food/insurance for the guide. I can't remember if that includes the airfare for the guide to Lukla. But the locals pay a much lower airfare than foreigners. I would not count on finding a guide in Lucla. Most of them are porters who may not speak much English and do not carry insurance. As you hire guide, also make sure you get your trekking permit taken care of (photos needed)before leaving KTM. Trekking agent can do that for you very easily.
尼泊尔的徒步向导大致分三类,一类是纯粹的向导,这类只管计划行程, 安排食宿, 介绍风土人情,英语大多说得不错,但不背包, 价格最高;第二类是背夫,只管背包, 一般都不太能用英语交流, 价格最便宜。第三类是向导兼背夫,这一类会用英语交流,也帮着背些东西。对大多数不跟团的徒步者来说,第三类无论从价格还是服务,都最实际。。需要说明的是,尼泊尔的社会等级制度还是挺分明的,据说第一类的向导多半是不会帮你背包的。向导还有个作用 - 在旺季徒步线上住宿紧张时,他/她可以先走一步,提前帮你定下当晚的住处。
2. If you plan ahead and schedule guide interviews before leaving US, you should be able to nail down a guide once you get there. Tour group have a set schedule, which could be problem for accllimatization to high altitude. From what I learned, those in tour groups have more problems with altitude sickness than those go on their own with their own guide as you can go at your own schedule. If you don't feel good, you can stay at a place longer to accolimatize.
There is also the social aspect of trekking with you own guide vs with a tour group. You definitely have better opportunities to interact with the locals with your own guide. A US based tour group means you will hang out most of the time with Americans and I don't think that's what you are looking for going to Nepal.
3. You probably don't want to negotiate too hard when looking for a guide because the guide agent will further cut into the guide's salary. Or you could negotiate hard, get a good price but give your guide a good tip. I would not book a tour from US either, as most of the money goes to the tour company, not the guide.
4. I didn't trek EBC; so can't speak about that. From what I know,generally with food the higher you go, the more expensive. On my ABC trek, I only paid for shower once at couple of $, as I recall. EBC is probably different. On EBC trek, you probably don't want to take showers anyways at high altitude, as you can catch a cold which could make altitude sickness once.
5. As a female travelling solo in Nepal, I felt very safe, definitely safer than Detroit downtown and Oakland downtown, for example. Often times I took buses to outskirt towns as the only gringo and I felt perfectly fine. The touters in Thamel are annnoying but not dangerours. The danger comes more from altitude sickness than crimes. That said, common sense still applies. One problem with us coming from US, car based culture, we tend to let our guard down, especially when you are tired.
6. In KTM, you should be able to find a decent hotel at $25ish (last year price), breakfast included. It's budget.
Happy travels and have a great time in Nepal!