Assuming a normal election where the pendulum swings back and a normal transfer of power (no certain things), the outgoing President could pardon everyone from his chiefs of staff and cabinet officers to low-level federal law enforcement on the way out, and then the Democrats would wring their hands and say there's nothing they can do, bar some large scale political realignment where Republicans lost control of numerous state legislatures and governorships by a large margin, as well as in Congress.
The best opportunity for a major restructuring of the legal environment would bea Constitutional Convention, but because Republicans have pursued this as a strategic goal for a while, Democrats invested all their relevant energies in being against it rather than developing any kind of strategy of their own, guaranteeing that they would get rolled if one actually took place because they went in with wholly defensive mindset and no plan to win. The fundamental flaw of the modern Democratic party is that it sees itself as a vehicle for competent management of the status quo, not a force for implementation of its voters' political aspirations. Thus is pays lip service of what its supporters want but operates to dampen and delay those same supporters whenever it gets into office in the name of continuity and responsibility. It operates on a combination of political rent seeking and fundamental conflict aversion.
This is why I find myself increasingly impatient with self-styled moderates. Wanting to talk things out and compromise is good, but it only works when there is mutuality between counterparties. When the political opposition is indifferent to questions of truthfulness or corruption, moderation degrades into appeasement; moderates will sell out their own supporters in the name of peace and quiet, while giving away the strategic initiative over and over. The previous Trump administration engineered a mob overrunning Congress in an attempt to stay in power, and only failed because the Vice President declined to aid the scheme; a mistake the current one surely doesn't intend to repeat. The incoming administration spent a great deal of energy prosecuting every footsoldier they could find who set foot inside the Capitol, but shied away from going after the people who actually organized it. The results speak for themselves.
Why it is even accepted that pardons can be issued before conviction?
Yes, this also seems bonkers to me.