there are hundreds of other websites apart from MSO that have information on AMC/USMLE
this site is mainly for Australian high school students trying to get into med, and doesn't have much apart from admissions
But to answer your questions AMC and PLAB (UK) are definitely more clinically focused and the assessment is less rigourous compared to USMLE
I would only suggest starting AMC MCQ/clinical in your clinical years, pre-clinical will only cover USMLE step 1
AMC MCQ is comparable to USMLE 2CK, USMLE 1 is in a whole league of its own as it is very basic scienced focussed.
Usually, UK/EU citizens that leave the UK to study medicine in other EU countries are on equal standing to a UK local graduate, and they can go back to the UK and get GMC registration seamlessly. I don't think this applies for EU medical students who do not have EU citizenship, and with this whole Brexit thing going on I am not sure what things are going on in that side of the world...
For an IMG, I would recommend doing the USMLE and getting ECFMG certified as the USA is more IMG friendly (i.e if you do well in it, you still have a good chance at matching to family medicine or internal medicine). Statistically, about 50% of IMGs match into residency programs, obviously depending on how well your scores are (particularly step 1), research, rotations/US experience, letters of reccomendation etc.
In Australia, the AMC is pass/fail and no matter how well you do if you pass you are still in the same boat, its hard to find a job, and even if you do it will be non-training with an unclear path to accredited training/fellowship. whereas in the US you match to the speciality straight away.
American board certified specialists are also in a decent position to come to australia and get the local fellowship through the specialist pathway, as the training is similar (albeit a lot shorter). but this pathway is case-by-case basis and its still very difficult no matter which country you come from. also, policies are changing all the time and australia is really looking towards training their own specialists rather than importing from the specialist pathay.
pm me if you need any more details