The main problem I see with the umbrella movement is that it isn't winning over the mainland Chinese. For the movement to ultimately work, it needs to become China-wide at this point.
It's really hard to tell. One can't speak openly about such things in mainland China so the only two possible options are: ignore the issue or come out pro China. Who knows what people actually think in such a situation?
The strong pro China sentiment shown among Chinese students abroad is also not necessarily representative: for one thing, they come from a monied middle class and don't represent the whole of society on the mainland. For another, I've heard that their activities get monitored back at home and they won't want to cause social credit problems for their families.
There are various videos and social media posts from mainlanders saying (anonymously) 'in fighting for Hong Kong, you are also fighting for us', and similar sentiments. Surveillance society runs so deep on the mainland that people don't have the chance to object.
I've also personally met people from the mainland who have visited Hong Kong to see it with their own eyes, as any reasonably intelligent and educated person knows they can't trust the media. Certainly people who have travelled abroad and kept their eyes open has had the opportunity to question why the representation of facts in Chinese media is so different from in other countries. I also have friends from the mainland who got the hell out as soon as they could, of course.
As to what proportion of the Chinese populace these people represent, it's hard to tell. It could be one percent of one percent.
There are similar but contrasting generational issues in Hong Kong and China. The young generation of Hong Kong, which the protestors/rioters are mainly composed of, grew up in a peaceful and secure time. The final days of the colony were apparently pretty rosy, but the rest of its history wasn't, with notable anti British riots in 1967 because of the hardships, as well as fighting when the new territories were given to Britain in 1897. Young people won't remember this: their grandparents might. This might be a reason for a generation divide in Hong Kong politics right now which has seen some families not even talking to each other while living under the same roof.
In China, the young generation has also grown up in a time of prosperity. They won't remember (won't even learn about, lol) the cultural revolution or the great leap forward - arguably the worst human atrocities in history. But again, their grandparents might, from first hand experience. There might well be a few grannies and Granddads out there who don't absolutely support the party but know better than to talk about it.
This is not to mention the many members of ethnic minorities in China (of whom the uyghurs and Tibetans are the best known) who may have their own reasons not to love the party.
It's a more extreme example, but if you met someone in North Korea, would you expect them to give you their honest opinion about issues there?