Chronology Protection Horizon (CPH) and sC (universal superluminal invariance)

In my world Hoshino Monogatari, superluminal (sl)-drive exist but so do Chronology Protection Horizon (CPH) and sC, which are emergent phenomenon from the various set of energy boundaries and conditions in the lower dimensional CFT space

CPH's role is straight forward enough, preventing the formation of a Closed Timelike Loop (CTL) by imposing a hard sl-speed limit depending on the circumstance, but without sC, CPH can lead to some very non-sensical result like as follows

Setting up a Minkowski diagram, the 0° line denote space axis, 90° denote time axis, 45° line denote c, subluminal speed correspond to a line between 45° and 90° and superluminal (sl) speed correspond to <45° worldlines

Given point A and ship B, B depart A at 00:00 at relativistic speed and at 01:00, B send the ship B1 back to A at sl-speed

Now common sense and CPH suggest that B1 can not arrive at A before B depart at 00:00. suggesting a speed limit Vb, so far so good

Now since B1 also have the relativistic v inherited from B, from A's pov, B1's top sl-speed, not accounting for CPH, (let's call this mV) is much higher while from B's pov, mV doesn't change at all

If mV exceed Vb, then Vb take seniority and B1's new top sl-speed is Vb instead, but since Vb is the sl-speed at which B1 could arrive at A right after B depart at 00:00, from A's pov again, B1 could arrive in an instant after B depart

It's 00:00, just one minutes after B depart A when B1 arrive back at A, and now B1 seek to head back to B

B1's 1st jump already impose another CPH-field that impose another sl-speed limit Vb1 directly correlated to the sl-speed B1 travel at in its 1st jump (let's assume Vb for convenience), so that B1 can't arrive at B before it depart B at 01:00

This present a potential problem, if Vb is too fast (say correspond to a -60° line), Vb1 can be too slow (to ensure B1 can't arrive at B before it depart B at 01:00), and what if Vb1 (say correspond to 60° line since Vb1 is over the same distance as Vb, just backward, and respect B1's CPH) slip below c (45°)? This prohibit not just B1 but also light from A to travel to B at c too, which is clearly nonsensical

This suggest that there is another mechanism conserving relativity in parallel to CPH conserving causality, a sort of fundamental sl-speed limit sC that take seniority above all, capping all superluminal travel to below sC and lower depended on the local CPH, to ensure that the sl-speed limit imposed by CPH on the return (in this case Vb1) can't be <c

(In my (very much unqualified) opinion, sC might be the -45° lines on a Minkowski diagram, forming a nice symmetry with c which is represented by the 45° line

So for example in our case, using angle on a Minkowski diagram to denote speed, since all sl-speed is below sC (-45°), Vb would be anywhere from 45°(c) to -45° range (sC), limiting the Vb1 (which is the direct opposite of Vb if you think about it since it cover the same distance from A to B, just in the opposite direction) to be in the 45° (c) to -45° range (sC), hence >c and no causal violation)