I will say I do think people tend to overpatronize the Qing dynasty and play up its isolationism, when it was actually
unprecedentedly open to European traders. The Macartney mission was rejected, sure, but a lot of things really screwed the Macartney embassy's diplomatic pursuits. In a broader context, it came after it was perceived Britain violated restrictions, quite a few fuck ups with the attaches to the embassy lying to the Emperor to overplay English submissiveness, and Britain's demands being not exactly
nice. The Emperor was particularly incensed by Britain asking the right to own and settle an island off the Chinese coast. Economically, there also wasn't an incentive for it from the Qing perspective - as the dynasty had undone the prior tribute and embassy requirement for trade with China, and Britain wasn't really offering a good the Qing wanted.
Not without totally co-opting the intellectuals of China so they identify with the Qing instead of looking toward the Ming or getting swayed by the idea of republicanism. The foreignness of the Qing made them an easy scapegoat for why China failed in the 19th century.
Possibly, but I could see it becoming incredibly conservative and only doing the bare minimum. It probably would be less likely to end up in a republic though barring a communist revolution and could look like the Shah's Iran--a corrupt, nationalistic, and repressive nation.
I think this is a bit of a chicken and an egg. The Han rejection of the Qing comes more out of the Qing dynasty's decline and collapse, rather than being an inherent cause for the collapse of the Qing dynasty. During the High Qing, you saw Han advisors and intellectuals utilized and relied on by the Qing dynasty. There was definitely an existing tension, but not nearly one enough to inherently doom the country.
Would a Han dynasty, had there been one during the 19th century instead of the Qing have been more popular? And therefore might have been able to push through modernising reforms easier?
You'd need a Han dynasty interested in reform. The Ming Dynasty were heavily invested in conservative - practically reactionary - thought, going so far as to ban all private maritime trade - something which would most certainly cut them off from modernization if repeated.