Where is the demand going to come for in Q4?
The same way the demand came in for the previous quarters. Q4 of a certain year does not change that. However some years have seen Q4 have a buying spike outweigh the selling spike.
Will retail, sans lockdown money, be able to shovel enough cash into crypto this time?
Spending choices differ among people but retail accepting crypto is still a much lower number outside the US. No specific answer for this but I can say that the effect of lockdowns have not been significant on crypto for the time being, but bad for the fiat economy indeed.
Enough cash or not, someone will buy and someone will sell, so expect to see prices going up and down and if you are cunning enough, you should attempt to take advantage of that.
Otherwise, it seems to me like upward price action will depend on institutional investors.
Institutional investors are not long term hodlers. They are looking for methods to hedging their assets. So you cant say they will help raise the price, they will as long as they know the reward is high and the risk is lower, but like I said, they are not long term hodlers, they will sell before you know it, because they are in it for the profit.