Yeah, kind of failed on the question part of it. Title was written before the post, what was written drifted away from initial intent.
Subsequently digging through the archives elsewhere, I see someone else pointed out the 10th Sui'kun issue a looong time ago, and
it also went unaddressed at the time. He however, didn't acknowledge that a Deity had been meddling in Tarrin's head shortly before then.
In the Firestaff series, which was written well before
Axe of the Dwarven King, IIRC Spyder was the one who said (To Tarrin and Jenna) there were going to be 10 Sui'kun after all was said and done. 7 for each of the elements of the weave, and 3 as backups and work that needed to be done "outside of normal channels" or something to that effect. She specifically named herself and Tarrin as 2 of them, and at some point in the course of the Firestaff series, Jasana was named as the one who would be the third "spare" as it were. I'm fuzzy on if that happened at the same time, or later on. In either case,
Tarrin was aware of this.
Which makes what happened in Chapter 18 of Axe of the Dwarven King and after that a bit odd. He knew there were going to be 10, so why did he suddenly start fretting after the 10th one turned up? Divine Meddling certainly accounts for how/why that happened, but it is mildly annoying that Fel probably deliberately added in the plot element,
as I'm sure that shift was deliberate and not an error on his part, but it then is left hanging out there in the breeze and never seems to be addressed afterwords.
In some respects, I can understand how/why that happened, at least with what is written. Tarrin(either one of them) realizing one of the Elder Gods of Sennadar tampered with his memory in such a way would be likely to have unpredictable results to say the least.
Yes, it set in motion a chain of events that
needed to happen, but the nature of how it was done would still be a major issue for him IMO. An alternate option is that The Overgod told Tarrin after all was said and done, and kept Tarrin otherwise occupied until after Tarrin was given plenty of time to cool down.
Besides which, with 9 Sui'kun in Niami's service besides Tarrin, there was no urgent need to replace Tarrin until after his situation was fully resolved. It took the Breaking for the other gods to agree to allowing Niami the "3 spares" she was given, and until Tarrin's situation was resolved, I doubt they were going to create any scenario where she could possibly end up with
4 spare Sui'kun. After all, they'd waited
several years to get to 10 after having reached 8. So while they agreed that having the spares was important, keeping the count at 10 wasn't an OMG emergency either. Or keeping it at 9 for that matter, as Jasana wound up over in Pyrosia with Tarrin for a time, which took the Sui'kun count on Sennadar down to 8 for several months.
Of course, with other comments made on that front(when establishing the new Tower on Pyrosia), evidently it may not be particularly critical for more than "a handful" of the Sui'kun to physically be on Sennadar. Their simply being alive and "out there somewhere" might be enough for
most purposes... And given Tarrin's unique circumstances, his specific location probably matters even less than it does for any of the rest of the Sui'kun when it comes to being connected to Sennadar's Weave.
So basically, the Gods had no intention or need of replacing him until he was either truly and permanently dead, or otherwise "fully disconnected from Sennadar" by some means.