The OP was very clear about how it was purchased, and subsequently maintained. 50/50 down the line.
No need to worry about any lending bank as the OP clearly stated there is no mortgage.
I see there are (at least) four things to watch out for with the strategy.
a) The step up works only if your mother holds on to the property at least for a full year before anything unfortunate happens. If not the whole exercise fails.
b) There is always a risk that your mother might change her mind and leave you out of the will either intentionally or unintentionally. Once you transfer the property to her name it is hers to do as she pleases. If she enters into a questionable con-deal like reverse mortgage or any one of those scams, there is literally nothing you can do. The only silver lining would be you wouldn't have any capital gains to worry about!
c) All the "benefits" such as mortgage interest (if one is taken out at a later time) and property tax deduction etc. from the time of your gift till you ultimately receive the full property will accrue solely to your mother. Also, She would be fully responsible for all costs as such, although you could continue to help out. Any such payments would be considered 100% gifts in the year given.
d) As you would be gifting a property worth $400K, you would need to take it out of your life-time gift tax exclusion. But I suppose that shouldn't be a problem with the current limit of $5+ million.
I see b) as the main issue you need to be at peace with. If okay with all the above, consult a CPA and go for it.
one additional thing came to mind after posting. Your county assessor may treat this as a transfer of property and jack up the property tax rate. Prop 13 in California protects you from property tax spikes only until a transfer. Check with the county office how they would handle this before you take any action.