Perhaps your concern about stimulating demand is explained by the broken window fallacy. The fallacy is the belief that there is a social benefit to income that is concomitant with a loss in a stock of wealth. Instead of using a stone to break a window you want to use a robot.
You said "demand was so low because of automation". I think automation increases valuable output so it increases income so it increases demand. I think you suggested a nonsensical premise.
I think current day robots are economically viable since there is a market for them. As @Fizz correctly says they use spare parts and they also use energy, usually electricity. Presumably they produce more value than they consume, otherwise nobody would buy one. What is a robot and what is the owner of the robot are hard to separate since robots do not have what I might call autonomy under the law. They are chattel. As chattel, you might say it is the combined unit of the owner plus robot that is the consumer and the producer. The owner might be a corporation. Robots can mechanically pay for goods and services but since it's not an autonomous individual it is really the owner that is paying, using the owner's money. The robot itself does not own money.
If the law changes to recognize robots as autonomous individuals, not chattel, not slaves, then their individual output would add to GDP and their individual consumption of goods and services would add to aggregate demand. They already do add to these economic measures, but as part of a combined unit of owner plus robot.
You asked if robots can replace human consumers. Yes, robots as autonomous individuals can replace humans as consumers of goods and services. When a robot as an autonomous individual consumes a rival good, a human cannot consume the same. I think this is easy to understand if we consider electricity. If a robot as an autonomous individual runs, swims, and bicylces a triathlon, it will use energy that a human could not have used to air condition their condominium.
Link to a description of rival goods.
You have edited your question so I am posting an addendum. In this addendum I will assume that the law changed to recognize robots as autonomous individuals.
If elite consumption increases by one dollar and consumption by non-elites decreases by two dollars, aggregate consumption will decrease but that is a transitory problem. The lower level of consumption will soon be matched with a lower level of output. Nobody will persist in producing goods that will obviously fail to sell. I sense your question is about permanent effects and you did not specifically express a concern about transitory problems. One reason it is transitory is because when people stop producing goods that do not sell they switch to producing something that does and when that happens the economic recession comes to an end. Consider that there is the "permanent" phenomenon of productivity improvement which will eventually cause income and consumption to increase for elites and non-elites and so the lower level of non-elite consumption is temporary.
You asked if a robot consumer will "keep capitalism afloat". If the non-elite, motivated by a temporarily lower living standard, desire to vote to switch to a non-capitalism regime I think the robot consumer producer has an uncertain effect in that aspect. The robot consumer will consume some rival goods while also producing some rival goods. In the long run the robot producer consumer will be powerful and it will produce much and consume much. They will be part of the elite. Under a progressive tax system human elites and robot elites will be taxed at a higher rate than non-elites. It might be the progressive tax system, not as much the existence of robot consumer producers, that helps the non-elite.
Since I am assuming robots are recognized as autonomous individuals I will not answer the questions involving owners of robots and robots being sold and rented.
You seem very concerned about the cost to build and operate robots. These costs are always decreasing because of the permanent phenomenon of technology improvement.
You seem to persist in being concerned about stimulating demand. I refer you to the beginning of my answer that said low demand because of automation is nonsensical. We do not need robots to stimulate demand of rival goods. The destruction of rival goods is sufficient to stimulate demand.