Do Chinese people have lack of homeland ties such that their applications are 'commonly' refused?
First, let's have a look at the statistics as taken from Visa Statistics from Consulates 2013 and filter it for China...
...and it indicates an overall refusal rate of about 4%, or roughly 1 in 25 applicants were refused versus about 96% were successful. That's about 1,500,000 Chinese visitors for 2013. There are comparable figures for 2014, but these have not been formally admitted into the EU archives yet.
So there is nothing in these figures that indicate a prejudicial treatment towards the Chinese on face value. Referring to the same sheet, we can get the figures for Sweden...
...and see that Sweden has one of the higher refusal rates (about 8%) of the 24 members reporting their statistic.
From these we can conclude that Sweden is a tougher member than most of the others but there's nothing to indicate that the Chinese are subjected to prejudice one way or the other.
Finally, looking at the two Swedish consulates in China that accept Schengen applications...
...the refusal rates are broadly in line with Sweden's overall refusal rate. So your assumptions (i.e., "commonly rejected") are not supported on a broad level.
Your question then distils down to the individual reasons for each of the people who were not successful. We only know from empirical data that many refusals are caused by the lack of apparent ties which is in turn caused by genuine cases where the applicant lacks sufficient ties AND applicants who had sufficient ties to their homeland but failed to make this clear in their application.
On to your specific questions...
Is it likely that the migration agency will question her ties to China
and refuse her visa application because they think she might not
They will absolutely examine her ties to China and if she does not qualify they will refuse. This is a matter of law and they are required to refuse when an individual does not qualify. The onus is on her to show that she qualifies (see Should I submit bank statements when applying for a UK Visa? What do they say about me? for any applicability it may have to visa assessment within the Schengen regime)
I have found lists of required documents for her application on these
websites. Should we include additional proof of her ties to China in
her application or is it enough with the required documents? If so,
what kind of documents could help?
The required documents are usually sufficient but if you have something of particular significance, like an employment contract with the Chinese government, then by all means include it.
Is it a good idea to buy flight tickets in advance and include copies
of them in the application? (I guess the return ticket signifies
intent and economic feasibility of returning, but it would be a big
waste of money if the application is refused.)
You need to read the Required Documents page again, where it says...
A booked return ticket (we advise you to delay payment until your visa
has been granted). Please note that this is not required for visas
for business purposes, short professional training, sports/culture,
and visiting family/friends.