Hi... I have a few answers for you -- below each section (I was an expat on L1 with my DH & kids joining me)
Working
Will my wife be able to work as a teacher in California?
What will she need to provide?
It will take at least 3 months to obtain a "work authorization" for your wife, which will be tied to your L1 visa status, I think it needs to be renewed every 6 months or a year. Some employers for degreed "white collar" work do not like hiring people who do not have permanent status in the USA. Moreover, I believe the teaching status will take additional certification / verification for her.
I would recommend getting early childhood education courses underway in her first few months, before she can work, then apply at the private preschools or daycares with educational components.
Finances
As far as I can tell no German bank will allow me to keep my ETF invested in Germany, due to strict financial rules from the US, correct?
I was able to keep my canadian retirement and brokerage accounts, but they were frozen while I was non-resident. The financial rules are essentially that the foreign banks need to report accounts and incomes to the USA, and that is an administrative burden, not to mention against privacy of their german customers, so they say no. Asian banks say no often as well, for this reason. Double check if this applies for L1 holders, or only permanent residents, however.
Bringing over my money from Germany and then being able to enjoy the benefits of Vanguard should be no problem right?
What do I have to keep in mind when transferring my money over the Atlantic? Currency fees, transfer fees, etc.
Research a foreign exchange house to exchange large currencies, you may save a few percent over bank rates. Should be no problem to open any us account once you have "resident" status. Easy Peasy.
I will have to start understanding 401k and roth I guess?
These are actually pretty easy. The big thing to watch out for is that the 401k has a 10% early withdrawl penalty, so if you return to Germany in a few years, your US trading accounts will likely be frozen (with most banks), and then you will eventually need to pay the 10% fee to access your money / bring back to Germany in future. Germany might allow you a tax credit to offset this, Canada did not for years. There are at least 2 banks in the USA that allow non-residents in the USA (canadians, anyway) to hold accounts, but most refuse due to the extra reporting and risk, and they can change their minds without notice.
What bank would mustacians recommend I go to for my day to day living?
Getting a credit rating will likely be annoying?
Credit cards will have to be got and used?
To get credit rating and banking -- I used the bank that my company wanted me to get a mortgage through (they were funding a step up mortgage), so they cleared the way for me to get a credit card.
My DH however, had to get one of the nasty cards with a $80 per year fee, put $1000 up front for a $1000 limit, have $19% interest and overage / late charges. This lasted for a year, when the next $80 fee kicked in before he was able to get a more "normal" card. His equivalent FICO with the Canadian branches of the credit agencies was over 800, but it did not transfer.
So, First get a checking account at a major bank, and get a credit card with them. Bring a letter from your employer, if you can, to smooth the way. In 2 years, you will have awesome credit rating (from zero), and there will be no barriers.
And yes, it is pretty much ESSENTIAL to get a credit rating established, (rental cars, utilities, phone, hotels, etc) so focus on that for 2 years, after which, you can stop using them if you like. Visa Debit cards work OK from the bank accounts in the USA.
When can and should we apply for social security numbers? From what I understand we cannot work without one right?
Upon entry to the USA, you will bring your L1 application letter as you cross the border agency. This will take 1- 3 hours. When you arrive, the very first business day, before working, you need to go to the Soc. Security office, with your L1A visa and make the application. You should get your number right away. Take this to your employer for the I-9 form to be completed (all new hires in CA need to have the I9 completed to show legal work status), and payroll set up.
Your wife will need to make an application for "work Authorization" -- I think we did it by mail.. anyway, it came in the mail about 3 months after applying.
Miscellaneous
Will mustacians in LA accept us in their esteemed circle?
I moved outside of San Francisco, and there was a healthy Expat community, and I mingled with a lot of German, French, Brazilian, Asian, Indian expats, in the usa for 2-5 years at a time. I found americans pretty receptive, open and accomodating, especially when you look somewhat like the person you are talking to, and the hispanic friends I made at work were especially nice to my family.
Are there any things I should keep in mind when negotiating with my employer?
-See if they will put you into temporary paid accommodation for 3 months, and
-provide a letter to a major bank to help you get a credit card, and a letter to help get a rental apartment with a USA credit rating. Rental leasing is typically for a year, and you will want to know about the area you are going to live in before committing.
-Negotiate for 3 weeks vacation. Many US employers try to insist on everyone getting 2 wks as a new hire, regardless of prior experience.
-If you are looking to move permanently, then ask them for assistance with fast track greencard application after 1 year in the USA.
-Ask for tax preparation asssistance for your first year, when you need to give tax returns to two countries.
The other item that surprised me in the USA was that retirement benefits, such as pension $'s did not vest for 5 years at my former company. That is a very long time to work there. So ask about benefits and vesting periods.
Will I need to open carry, clap when coming across green traffic lights and give exorbitant amounts of tips everywhere?
California has more laws about legal handguns than other states, Do not worry about open carry, etc. But yes, there are many non-legal weapons out there.
Tips -- 20% for restaurants, always have a few dollars on you for shuttle porters that help with your bags, attendents at hotel free continental breakfasts, or anyone else who you see putting in a real effort to provide good service to you. Chances are at least half of their salary is tips, unlike at home. I never carried cash at home, but found it embarrassing to not have tip money at hand in the USA, usually in my pocket.
Will Trump let us stay should he become the next President? He won't I am reasonably confident in that.
I don't think Trump can do anything about it, even as president, once you are legally in the USA... BUT.. watch out because green card (permanent resident) processing or L1 renewals can get shut down or restricted by the federal or state side governments. It happened while we were there, and realizing that it would take another 3 years to get green card status made us finally pack up and leave the country.
What other questions
I was surprised that completing taxes myself, in my second year, that I had to complete 3 tax forms - federal, state, and ATM calculations. Not fun. i only had one set in my home country.
Second -- how mortgages work in the USA was a lot different from home, even when the ads actually read almost identical.
I was surprised how few people at work grew vegetables, went hiking /camping or other nature-like outdoor pursuits. Some do, but many were too busy with long days and long commutes. i had assumed that California sunshine would make everyone relax a bit more than what I saw at work.
Good luck! Relocating without children should make this an adventure for you.
The hard part will be finding a place to live that you like / afford, then for your wife to find work. Once you have that down, I am sure you will love the experience.