Your post may be hinting at two distinct problems.
First, saddle fore-aft position
You seem to want a more forward position than you can currently achieve with the bike and with the seatpost you have. I'm not sure what model of seatpost came with the bike. Seatposts can come with setback, i.e. the clamp is positioned rearward of the post's centerline, this parameter being measured in millimeters. The picture below, from Cyclingabout, shows a setback post on the left and a zero-setback post on the right. All else equal, a setback post does offer more comfort as the post can flex. Typical values for offset are 20-25mm.
Then, let's consider the bike's seat tube angle (STA). Traditionally, bike manufacturers have offered steeper STAs on smaller frame sizes. Presumably this is based on anthropometric data (i.e. data on human body proportions, hopefully accurately measured on a random sample or at least on a sample representative of people as a whole). This is one of my personal beefs with Cervelo: all their previous frames had a 73 degree STA. I assume this enables them to use one rear end across all sizes in the manufacturing process. That is, for many molded carbon frames, the front end and back end are manufactured separately, and then bonded together. If you use the same size rear end across all frames, that's fewer molds to make.
STA and seatpost setback may intersect with your own desired amount of saddle setback: the horizontal distance from the tip of the saddle to the center of the bottom bracket. This measurement varies depending on the length of your saddle. Mine is basically zero with a Fizik Antares saddle. I don't believe this is typical at all, and most riders have at least some saddle setback, I think at least 50mm. In any case, it's possible that some seat tube angles and some values of seatpost setback (common values are 0mm, 20mm, and 25mm) may preclude you from achieving your desired amount of saddle setback. For my case, I may be unable to achieve my desired saddle setback on a Cervelo even with a zero-setback seatpost, or else the clamp might be at the edge of or slightly outside the allowable clamp positions on the saddle rails. You stated that you have long legs and a short torso, which is the opposite of my proportions. I would normally expect long-legged riders to want relatively rearward saddle positions (but preferences vary).
Right now, let's assume you actually want less saddle setback (i.e. you're doing this to achieve your desired pedaling position, and you're not doing it only to shorten the reach to the handlebars, discussed in the next section). In theory, if you put a reversed Thomson Elite post on the frame, your weight will be biased relatively more forwards on the bike. That probably changes the handling characteristics of the bike. It may be twitchier (i.e. faster to turn) than you would prefer. It may be that you can adapt to this. It's not really possible to say without trying it. If you are not on a zero-offset seatpost right now, you could consider trying one of those first to see if you have enough horizontal room to achieve your desired saddle position. Anyway, as someone mentioned in comments, in the past many triathletes would use reversed seatposts on standard road bikes so that they could achieve a reasonable aero bar position.
Second, your reach to the handlebars
First, let's mention the concept of frame reach, which is described in a road.cc article and illustrated below:
Frame reach is basically how long the frame is, isolating the horizontal dimension and measuring from the bottom bracket. Once you have set your saddle's fore-aft and up-down position to suit your leg length and preferred pedaling style, you should only then adjust your handlebar position using stem length and height. Secondarily, you can choose handlebars with shorter reach.
Anyway, after setting our desired saddle height and setback, we then change the stem length to achieve the desired distance to the handlebars. The frame reach, plus your saddle setback, plus your stem length and height all alter the reach from where you are sitting to the handlebars - or, more specifically, to the center of where your stem clamps the handlebars. You can further modify your reach to the brake hoods slightly by choosing handlebars with a shorter reach, and to some extent by altering where your brake levers are positioned on the curve of the bars (but be aware this alters your wrist angle when you're holding the hoods, and you want it to be neutral, plus you can affect your ability to reach the brake levers in the drops).
All else equal, a shorter stem will result in faster handling, but this difference is secondary to other factors unless we are talking about a really short stem (e.g. 80mm or shorter; most road stems may not even be available under 80mm in length). If you are fundamentally on the wrong bike size and you can't shorten the stem further, you may have to compromise by moving the saddle forward some, but you're still on the wrong size bike. I recall hearing from one bike fitter that cyclists with relatively short torsos tend to be harder to fit to a bicycle than cyclists with long torsos.
If you are shopping for a different bike, here are some considerations. I think that Cervelo's R-series can be classified as a performance road bike, and these tend to have long and low positions, i.e. relatively long reach and low stack. You want a frame with less reach. You may be better off looking for an endurance road bicycle, which is a less performance-oriented type of road bike. Within each subcategory of bike, frames also vary as to how long they are. You can use a site like Bike Insights to examine bicycles, provided the geometry is in their database. For example, they classify the Cervelo R-series as a performance road bike, and within that category, the bike has a somewhat upright position (i.e. relatively shorter length and higher stack). (NB: if you manually search this site, note that the proper spelling is Cervélo, with the accent mark over the second e.) Sticking to Cervelo since you're familiar with the brand, you'd consider a bike more like the Caledonia or even the Aspero (technically a gravel bike, can accommodate road tires). You also could consider looking outside Cervelo's bikes.
Last, if your physical proportions are very unusual, you might want to start considering a custom bicycle. Custom bicycles are not necessarily super expensive. However, if you relayed your bike fitter's statement correctly, your proportions might be unusual enough that you may have a lot of difficulty fitting to a stock bicycle. A custom bicycle, provided that you either have a good bike fit or the builder is a good bike fitter, can solve a lot of fit problems.
