Good question (however I hope that is not off topic). That is indeed a question which often appears in my mind. My background is in Thai Theravada Tradition, it is also in Suttas, and – moreover – in Ven. Nanavira Thera. And even that teaching sometimes seems to be different I do not have a problem with any of the aspects. I hope my following explanation will not misinterpret anything.
According to my observation 5 khandhas in Suttas are not completely the same as it is in Thai teachings. I think that also Thai language plays its part, for example, saññā is Thai would be memory, and sankhārā would be thinking (Ajahn Gavesako would know better since he knows Thai). But according to the Suttas, saññā would be perception and sankhārā would be determination. To interpret perception just as a memory certainly overshot the real meaning of the word and there are some limitations too. Memory is experienced as anything else with other 5 senses: so, when there is experience of something there are all 5 khandhas, when there is experience of memory there are all 5 khandhas too. So, perception must mean something more precise than just memory. The same would be with sankharas... But I will not go here too much into detail here.
In Thai tradition quite often rūpa is interpreted as this physical body, and in that way it equalize rūpa khandhā (form aggregate) with kāya (body). Again, according to Suttas rūpa is something much more subtle than just this body. According to Thai way, when we become detached with body we actually experience cessation of 5 khandhas (according to Suttas) as whole; we have full understanding of the nature of any experience of identification. Imagine: you do not identify this body as your self any more, your feeling towards it is ceased too, also your perception and determination (intention) are ceased too, and consciousness which used to be directed towards 'permanent, pleasurable' body is ceased too.
So, identification with such gross physical appearance (kāya) is ceased (sa-kāya-diṭṭhi), but conceivings of rūpa (and of other 4 khandhas) does not cease yet. And when Thai teaching say that sotapanna does not experience the cessation of mental part, I understand that as that sotapanna didn't abandon of more subtle aspect such as conceiving, greed, aversion, delusion, etc.
That this make any sense to you? The matter is much more complex, and I offered here only a brief summary.
With best wishes,
Bhikkhu Hiriko - ÑāṇasuciThe experts do not say that one is a sage in this world because of view, or learning, or knowledge, Nanda.
I call them sages who wander without association, without affliction, without desire.
The Buddha, Sn.V.8.2 (1078)http://pathpress.org