1. Generally speaking employer contributions get entered on the "Other Income" line. They are normally foreign source for foreign tax credit purposes; but do not qualify to be excluded as part of the $80,000 foreign earned income exclusion.
2. It will depend on the kind of investment. Can you be more explicit here? Is this a UK endowment policy, unit-trust ISA, stocks & shares ISA, national savings certificates, etc etc.
3. Stamp duty is not claimable as a real estate tax (nor is council tax). Mortgage interest that you pay is deductible, but gains made on repaying foreign currency mortgages are taxable as are gains on sales of residences. For these reasons it is common planning for the home to be owned in the name of the non-American spouse.
4. Do remember to file separate TDF90-22.1s for joint foreign accounts each year plus those for any sole foreign accounts.
5. The US return must report worldwide income and gains even if not taxable in the UK (eg child benefit, premium bond prizes, ISAs etc). The UK only needs worldwide reporting if you are domiciled within the UK, which is unlikely for most readers of this forum.
In order to protect yourself from penalties you may want to think about having the US returns looked over by a specialist before filing.