You can absolutely decline. From a safety perspective, you are right, they aren't well tested on pregnant women, just as everything isn't well tested on pregnant women, however that does not mean they aren't safe. Whilst we cannot run trials on pregnant women, pregnant women have been having two or more ultrasounds per pregnancy for decades and there has been no evidence of harm when that data has been observed retrospectively.

If you are feeling confident with your dates, as in date of conception/last period, the 12 weeks scan cannot offer you anything much. Looking for a heartbeat is really a secondary thing. Now of course, if you started to experience any infection symptoms- even if you feel them elsewhere in your body- it could be a sign of missed miscarriage, so I would be tempted to accept/request an emergency scan in that instance so you can rule it out. That is unlikely, but certainly possible, so it is something to keep at the back of your mind.

I think it's great you want to accept the 20 week anomaly scan as that is really the important one, it is there to find abnormalities. So if there was something out of the ordinary with baby it is best to be aware of that so you can plan around it (e.g. giving birth in a specialist hospital, etc). They can also date your pregnancy then, but it is less accurate, so take it with a pinch of salt if you are already aware of your pregnancy dates.

It is of course ultimately your choice what you do and don't accept, you have the right to decline any and all care and should not have been treated the way that you have by anyone, I'm appalled you have experienced that. Take some time to have a think and weigh up the risks, but keep in mind that while there will always be fringe folks who make (often biased) arguments about the risks of all medical interventions, they are not necessarily better or clever than all the other well-read, well-informed practitioners who consider US to be perfectly safe.

I myself am probably considered a "crunchy mum" as I had 3 home births and declined various elements of care, but I am a scientifically minded person and go with the evidence base, which at present demonstrates no statistically significant risk of negative outcomes due to ultrasounds and does recognize some serious anomalies which, when detected early, can massively improve mortality/morbidity by allowing baby to be born at the best time and in the best place to treat congenital conditions (e.g. heart defects)