For my US taxes, I include my employer and employee MPF contributions as part of my total earned income, and I convert my income to USD using the IRS' currency conversion table for the relevant year (I don't bother trying to convert whenever I make a contribution.) For 2020, I just used the 1USD = 7.756, which you can see at https://www.irs.gov/individuals/inte...exchange-rates
For FBAR FinCEN Report 114, I use the treasury's rates, and I do input my MPF accounts highest balances. This is annoying to calculate, but it's usually after my latest MPF contributions get into my MPF account in December, and I look up the fund prices (I just use two HSBC MPF funds these days). I didn't do anything when I changed my MPF investments because at least in my non-professional opinion I didn't take out the money and I can't actually get the money (until I permanently leave HK), so I shouldn't need to do anything (I'm hoping this is good enough for acting in good faith).
I will do everything I can to avoid filing Form 8938. Except for MPF (HSBC) and TVC (BEA) all my liquid investments are with US brokerages (Schwab & M1 Finance), and I've also never for even a moment had 300,000+USD in liquid assets in my name. I've also never had 200,000+USD in liquid assets in my name at the end of any year. My one and only property (a flat in HK) that I live in doesn't count towards the Form 8938 applicable assets.
You've asked lots of good questions, and I don't have answers that 100% will be IRS audit compliant. I'm always scared of the IRS and a potential audit, but I've generally tried my best, and I consider myself a small potato that just tries to do the right thing without paying tax professionals to do what I strongly believe should be free and fairly simple.
When it comes to using tax professionals, it's important to remember their incentives. Some try to be as honest and helpful as possible, but some want to make things as complicated as possible to charge you more fees (just like investment professionals). Personally, I try to avoid tax professionals and/or lawyers, but my avoidance costs me peace of mind when it comes to the IRS. I've used TurboTax since I got to HK in 2012 (I used Taxes for Expats for my overly complicated 2014 US tax return), and so far I haven't been audited, and I hope my luck continues forever.
I have no idea how to treat TVC accounts appropriately, but I did at least put my highest 2020 TVC account balance on the FBAR FinCEN Report 114. I converted my HKD balance to USD with the US Treasury 31 December 2020 rates at https://www.fiscal.treasury.gov/repo...istorical.html
HSBC doesn't allow US citizens to open TVC accounts, but BEA let me. Maybe HSBC doesn't want to deal with US citizen's TVC paperwork (they let me have MPF accounts, but I haven't found a way out of MPF for my circumstances). Or HSBC thinks US citizens aren't allowed to have TVC accounts, but just like asking two tax professionals, or two people from the IRS, or two US citizens working in HK - there is a good chance you'll get two different answers, so maybe just HSBC's tax lawyers said TVC accounts are not allowed for US citizens, but other financial institutions either didn't consult tax professionals or their lawyers gave them the green light for US citizen's TVC accounts.
Oh another thing I'd like to know how to do legally would be how to transfer my US investment assets to my wife (no US tax obligations of any kind) without paying any taxes or fees (less than 100USD). I don't want to sell any of my investments (I want to avoid taxable events), and I think Interactive Brokers HK would have all the ETFs I am currently invested in with M1 Finance (I just follow Paul Merriman's advice). Except for the 30% dividend issue, customer service, and ease of use, it'd be best to keep assets in my wife's name whenever possible. However, so far I've kept things mainly in my name as I try to build up my wealth (buy and long-term hold).