The Swiss Franc (CHF) tends to rise against every other currency (no exception I'm aware of), see for instance historical rates on fxtop.com. For instance the USD lost 2-4% per year over most multi-decade periods (even ignoring the '70ies on the grounds that exchange rates had been locked before then, and once they floated it took them multiple years to find an equilibrium price). That means your US stocks need to outperform your Swiss shares by 2-4% each year just to make up for that currency loss. For example if your USD stocks gained 100% over the 25 years you held them, and assuming the USD keeps losing 3% on average against the CHF, their value in CHF would be the same as you bought them for (ignoring dividends here to keep it simple).
In many cases you can buy ETFs that hedge currency risk, but that comes with a cost of its own (check the prospectus). No idea what's best, just highlighting a point worth considering.
1. You get withholding tax back by by filing a tax declaration and providing proof. If you're taxed at source I think there's a particular form, though I could be wrong. Ask your Steueramt.
2. No difference. But since you get less to (re-)invest, which comes with broker fees, accumulating is slightly preferable.
3. Under certain circumstances capital gains can be categorized as taxable income, but that won't happen if you hold for years. So no CGT in CH for you.
4. Depends on what you buy and how often, so whichever works for you. One advantage of a low cost broker is that switching also comes relatively cheap should you change your mind.
5. Not an option for 3rd pillar AFAIK, that typically comes with very few funds available all of which tend to not be very low-cost. Personally, I regard 3rd pillar as a part of the cash portion of my investments. For 2nd pillar the Pensionskasse does all the work, you have no say.
6. Open an account with them as non-US private investor. Just be aware that AFAIK that means you (your heirs) are subject to US inheritance tax upon your death (just saying). And there may be issues with their tax form, ask your Steueramt.
7. Yes, most personal wealth is subject to wealth tax, AFAIK only 2nd and 3rd pillar are exempt.
8. Only you can answer that. Have a tax declaration made and see what's better for you, shouldn't cost more than 200CHF if you're employed and not owning real estate.
9. Funds usually buy and sell "at market" price, for which there's a bid and ask price. By necessity there's a small difference between the two, it's called spread; the less liquid a security is the bigger its spread. As a consequence of a round trip (buy xy shares and sell xy shares of the same company) the fund loses the spread (because it sells at bid and buys at ask), a hidden cost hardly anybody talks about and that's not included in TER. The higher a fund's turnover the more often it is hit, and the less liquid the securities it invests in the harder it is hit each time. Therefore, and everything else being equal, the lower its turnover the better.
9b. Risk with respect to bankruptcy is the same for any ETF because every ETF is a legal entity of its own. As a consequence even in case of bankruptcy of UBS, Deutsche Bank, iShares, S&P, Lyxor, or whoever else may be the funds' sponsor or your broker, your money invested in any ETF is safe.
9c. Tracking error; usually is negative and means slight underperformance to the index. Usually less than 0.1% so not really worth making much of a fuss about unless perhaps a contender showed a positive tracking error.
For completeness sake:
Contrary to ETF, ETN are a credit and as such come with creditor risk, i.e. are at risk if the debtor were to go bankrupt.
Some (but not all) ordinary funds come with issuiung(?) and redemption fees of 0-5% each. Personally, I would never consider buying any that do, though unfortunately that can change anytime.