1. It defaults to US website once I logged in Avis Preferred. Will try to use other countries site but the residence probably would still need to be in US though... With Avis the card used in file has opted no currency conversion though 9 times out of 10, we still need to make sure it is billed in local currency.
We always opt for pay at the counter because our plan often changes, hence dont want to deal with cancellation / refund on pay at reservation, plus a penalty / fee may be involved.

But to our surprise once in FRA, even though the DCC clause was taken out (not shown on the final agreement and the agent made sure to show that to us), the bill was still billed in USD - to the dot of the same amount quoted on the reservation - which was not a DCC amount but regular market rate conversion. Euro was going up against USD between reservation and actual rental - yet our bill being in USD of the original market rate conversation made at the reservation time, actually saved us over $30 - that may be the only good experience coming out from the currency conversion!

2. and 3. Retired so are using the airlines / hotels codes. Hilton when it partnered with Avis, offered the best rates for rentals made in Europe. Unfortunately HH changed to partner with Hertz a few years ago.
Oddly the old code still works as not being rejected as invalid. Though it gives the same small discount as Visa Signature / BA Silver.

The Visa Signature discount is claimed to be 25% but it is not true. Only a tiny one, not even 10%.

4. Not elite on either program, hence it is risky to use the 2 codes.

Thanks for the suggestions though, especially on trying other countries sites. Will do that.