<p>I just re-ran all basic calibration steps from the Original Prusa i3 MK2 Manual.</p>

<p>Now, when doing the first layer calibration, lines that are running in positive X direction are ok, while those running in negative X direction are severely squished.</p>

<p><a href="https://i.stack.imgur.com/5zzEl.jpg" rel="noreferrer"><img src="https://i.stack.imgur.com/5zzEl.jpg" alt="Print of the default V2Calibration.gcode file on my Prusa i3 MK2"></a>
<em>(The "waviness" of my print bed is an artifact of the camera lens distortion of my smartphone)</em></p>

<p>I already did Bed level correction, so each line is exactly the same width over its entire distance and tried to raise the live-adjust Z, but that leads to the thin lines not adhering at all. My printer is 100% stock, I modified nothing about it. </p>

<p>What can I do to troubleshoot this further?</p>

No FDM print at all.
The problem of your design will not be the materials, but a basic property of FDM printing: FDM Printers do create a structure by placing a long string of filament next to itself and ontop of itself, creating tons of boudaries.
These boundaries between the layers are the weak points for this application: Even if the material like ABS could withstand the blow handled with such a club, the print will break at its weakest point - which in this case is any layer boundary. This is amplyfied by the basic design we have here: The elongated shape will serve as a lever on each of the weak boundries, until one gives way and results in catastrophic failure and a flying clubhead.
Non-FDM for the rescue.
To counteract this, you need to use a different method than FDM printing to get a more homogenous material than the bound deposited filament. Such methods could be for example SLA (Stereolithography) or SLS (Selective Laser Sintering). Both could easily offer even tiny details.
SLS uses Nylon or metal powders, sometimes even ceramics - Tungstencarbide for example.
SLA/Resin
Using a Resin printer using the SLA methods results in an object almost as homogenous as an injecion molded object. Proper aftercare and curing is required to get the best results. Also, Resin prints usually age under UV light, which can negatively impact lifetime. SLA printers are expensive (for home printers), print shops that offer them relatively rare and costly (in comparison to FDM) but usually offer superb resolution and almost perfect smoothness. A lot of the exact material properties is resin and aftercare dependant.
A way around the aging could be that the results of an SLA print could be used to create green-sand molds countless times, which can be used for casting metal or even some thermoplastics. Remember though, that cooling metal shrinks.
SLS Nylon
Nylon would be a medium rigid, light solution, but it ages and has a quite rough surface. It does offer some flex, almost perfect for this application. While most SLS machines for nylon are commercial to industrial, print technology of this kind is widespread enough to make them somewhat affordable (for an industrial printer) and printshops for these relatively common, prints are not cheap but well priced.
DMLS / SLM
Direct Metal Laser Sintering and Selective Laser Melting - an evolution of SLS - allows to create structures from various metals by sintering/melting powders of metal at the right spot to gain shape. The benefit would be, that you get a part that could withstand much more destructive testing than your bottle used as a clubhead - you get a workpiece of solid metal (Steel, aluminium, Titanium and a lot others are available) after all that has the same properties as a cast item. The big downside is, that only few companies currently delve into DMLS, among them the former patent holder of many of the FDM printing patents,
Stratasys
. This means, that a machine for this is industrial rated and priced, and that print suppliers charge accordingly.