I differ to you as you appear to be far more knowledgeable about the system in question than I. Could the Legislative Yuan have become a sort of ultra-small upper house type body or is that impossible with the nature of the body.
From what I can tell, probably not - the National Assembly was largely a unicameral parliament-cum-electoral college. So your latter assumption is largely the case.
It gets even
more complicated because both the National Assembly and the Legislative Yuan were popularly elected and whilst the intent was for the National Assembly to be the Parliament, there were always those who viewed the Legislative Yuan in the same way. In that case, rather than the Europarliament/Council of Ministers analogue I mentioned in my last post (and which was the intent, in theory, of how it operated), in reality it was more like the modern-day PRC. In that case, the National Assembly would be a cross of both the Political Consultative Conference and the National People's Congress, on one hand, where all sorts of proclamations are made but in effect largely rubber-stamps policies made by the Legislative Yuan - apart from its other existing duties, which were quite extensive (i.e. electing the President, being the sole body through which changes to the Constitution could be made, etc. etc.). Hence, the Legislative Yuan was a popularly-elected Standing Committee that did all the nitty-gritty stuff. This is why when martial law ended in Taiwan in the 1980s and Constitutional reform began in earnest in the 1980s and 1990s, the focus of being the country's Parliament shifted to the Legislative Yuan from the National Assembly (since the latter was seen as a useless rubber-stamp body - an upper house with no real power) and the National Assembly was ultimately abolished.
This shift in power was obviously
not was Sun Yixian and Jiang Jieshi had in mind, but that was the reality of how it happened. Had the ROC retained more provinces than just Taiwan and a few islands that made up the rump Fujian province (as well as, for a time, a few islands that made up the rump Chekiang province before that got conquered in the 1950s by the PRC), then the nature would probably differ. If given more time, there could be a possibility of things shifting towards a tricameral direction. Originally, Sun Yixian wanted to model the National Assembly on the Japanese Diet - this was during the late Meiji and early Taishô periods. If taken literally, then over time it could be possible for the National Assembly to be bicameral (a Senate modelled on the pre-war Japanese House of Peers and, to a lesser degree, the post-war House of Councillors; and a House of Representatives on the Japanese model), with the Legislative Yuan as the Standing Committee as per OTL. The end result of
that would be very interesting in an ROC scenario were more provinces were retained.