Is it permissible for an ordinary person to assess the excuses that allow one to miss congregational prayers, to rule on the permissibility of sharing intentions for intertwined acts of worship, and to apply rulings to issues based on the conditions and guidelines of scholars by himself, or is he obligated to ask people of knowledge regarding all of this?

The two issues are different. The excuse for not attending congregational prayer is left to the individual's own assessment of their illness that prevents them from attending. As for the permissibility of combination (of prayers) or not, it is a purely scholarly issue in which the common person refers to the people of knowledge. Thus, the matter entrusted to the individual is determined by them based on the scholars' guidelines, and what requires the words of the scholars must be referred back to them.