Interesting variety of responses. Sean, I applaud your ingenuity in reaching out to the Bigger Pockets group from the tenant's point of view. Here's my two cents' worth - sorry it's a little long.
You are the first tenant post-rehab, so the fact that you are "test driving" this for the owner should be a consideration in your conversation with the landlord. You've obviously gone to significant effort to document and examine the problem, which is a heck of a lot better than a tenant who calls and says "the heat doesn't work".
Evidently that landlord has made SOME effort, having sent an HVAC person there twice. I wonder what the outcome of those service calls was.
Looking at this from the landlord side, this is not just the tenant's problem. If this tenant has a seemingly legitimate issue with the house being cold, so will the next tenant, and the one after that. Telling the tenant to like it or lump it is counterproductive in multiple ways. Once the landlord is aware of the issue, it needs to be addressed if the landlord intends to hold onto the property. Otherwise, it will be a continual thorn in the side every year, and it has the potential to damage landlord credibility.
In your conversation with the landlord, emphasize that it is in both parties' best interest to diagnose and permanently fix this situation. You definitely have negotiating room with the landlord. At worst, you break the lease and he can keep your deposit and/or take you to small claims court. I imagine, however, he would prefer not to have an empty unheatable unit to market in winter, and still have the same problem to deal with next year with another tenant. There is motivation on both sides to find a solution. Terminating the lease (without mutual agreement) should be the last resort.
As far as space heaters, they use more electricity (higher cost to tenant) and pose some degree of safety risk. If you go that route, LL should be asked to pitch in and compensate in some fashion for the additional expense. Physically, the advice others have given, such as closing off the upstairs vents and sealing off the path to upstairs (if feasible) are good advice for maximizing the use of the existing system.
I try to keep my BP posts positive and informative. In this case, I must speak up and say I am disappointed to see responses such as "you are out of luck" and "this would be a comfort issue". Both of those comments seem like knee-jerk reactions to a tenant complaint. To the credit of those who made the comments, they both came back with "work with the landlord", which is a much more appropriate answer in this situation.
I'm confident the house was not advertised as having "some heat" (60 degrees) and the rent wasn't bargain-priced based on this defect. Working HVAC is a basic creature comfort in the 21st century in the vast majority of homes. To dump this on the tenant, even if you're legally correct (which I personally doubt that a judge in a tenant-friendly state would agree with), it's the wrong thing to do. This tenant has documented the insufficiency of the heating unit. Considering the short duration of his tenancy and the newness of the rehab, it would be hard to place this burden on the tenant's shoulders legally or morally.