Irrelevant, unfair, or inadmissible evidence is excluded. The jury makes a decision based on the totality of the presented evidence. Both 'sides' present their best case for consideration.
Plus see previous answers.

Plus the judiciary is not a very diverse group, so query effective representation of communities & protection from unconscious prejudice.

Whilst that may or may not have been the case it is largely irrelevant. MoJ research (2010) shows that juries do generally make fair decisions. They also ensure lay involvement/public confidence & can act as a check on govt power ( e.g. Colston 4)

What may or may not have been the populace/pool from which jurors were drawn is irrelevant. The principle of a fair trial, consideration of evidence by ordinary citizens & the arrival of a verdict by those citizens is paramount.

You are so wide of the mark. Questions posed at trial are based on evidence & documentation. All of which is seen by both 'sides', save in exceptional circumstances (national security)& both 'sides' can pose questions & re-examine on responses to those questions. A jury can also ask Q's of a Judge.

In addition lawyers in the UK do not 'game the system' & are actively prevented from doing so. Parliament, however may try to game the system by pushing through anti-democratic laws, as with this attempt & which need to be challenged.

Incorrect analysis. In 1220 & towards Magna Carta local officials or representatives, such as the king's justices or local lords, oversaw legal proceedings. There was then & is now a need to enable a fair trial. This occurs with Juries. Any competent judge/advocate can explain expert evidence.