Particularly with the leveling experience how can I become the best Ase Jr. on my path to 75? I will have PLD and BRD at 75 so Sword merits sounds like a start. MP for more spamming in parties. Once 75 what are people's choices for the groups?

Preemptive merits besides what you mentioned would be eva/parry str/dex/int/mnd, all great stats for blu

My recommendation when you ding 75 BLU is max BLU skill, sword if it wasn't already capped, then phys. Potency, then assimilation. Merits after that are mostly up to you. All group 2 merits have their uses depending on what you want to do. Convergence is probably the most situational, you could 5/5 Diffusion if your heart is set on tanking I guess. I am 4/5 enchainment and 1/5 Diffusion myself.

Cat 1 merits have a few meh items, monster correlation is very meh, burst affinity recast is also not great

On the subject of TP, last I tested it, CA and TP's relationship on damage increase wasn't granular and changed on a per-spell basis, has this been fixed with the string of updates that happened a few months ago? To clarify, the tests I ran were with some of the major spells that get increase dmg based on TP only got their dmg bonus at weird TP plateaus. For example, Death Scissors would do 600-700 dmg when used with CA and 0-749TP, it would then jump to 900-1000 dmg with 750-1499TP, and then jump to 1200-1300 at 1500TP+ and not increase in dmg beyond that.

That's basically how it works, consider every physical and magical spell you cast to be a weapon skill. They all have different mods and, in the case of physical spells, interact with tp differently. You can find the fTP values for some spells that were figured out online, not all of them iirc, e.g. your Death Scissors example:
0% : 1.5
150% : 2.75
300% : 3.25

You can see that Death Scissors is like an inverse Tachi: Gekko, and that TP above 150% is less powerful than tp below 150%, and the relatively slight increase was likely obscured by randomness. Just from my experience with having Enchainment, I haven't noticed plateaus at 0-749 or 750-1499. It's just the difference can be hard to see without doing it a lot due to randomness.

Well I had spent quite a few hours spamming CA skills on cooldown on the same mob type (camping NMs gets boring, sue me). While there was some randomness in the results, there were clear thresholds that radically changed the damage output of some spells. Problem was that 1) the damage values and thresholds were very far off of the wiki's values, and 2) not every spell used the same thresholds. Of course, this was done with less than one or two hundred CA casts per spell, so the same size was on the low side. But the results were significant enough to mark TP's contribution to dmg when used with CA to be contrary to the information gathered on the wiki and on various blog sites. Hence my original question: was this fixed?