In a PCT filing, is there any estoppel effect of arguments presented during the international phase (under art 19 or art 34) on national phase filings?
Yes. Japan (at least). see: https://aippi.org/download/commitees/175/GR175japan.pdf
In particular where it says: For example, the following may be taken into account for the purpose of claim interpretation and may be subject to estoppel: the description of the prior art cited in the application of the disputed patent (Tokyo District Court decision, December 22, 1986); an allegation made during the prosecution of the original application in the case of a divisional application; statements made in a corresponding foreign patent application; statements made during negotiations for licensing; and statements made during actions for an injunction or actions for a declaratory judgement for no right to demand compensation for damages.
As a general rule, however, no Examiner appreciates being told the opposite of what was told to another Examiner.
Under Art 19 and 34 of WIPO, for an amendment, a statement for basis of amendment is required.
I feel, if amendment is made to specification and/or drawing than the section of specification and the drawing prior to amendment and statements thereto may cause estoppel action because the later version is chosen as the best mode of practice and applicant is supposed to frame claims for protection of best mode of practice.
For claims it's a bit different. USPTO allows reinstatement of withdrawn claims and canceled claims (with new serial number). So, prima facie, amendment of claims and arguments thereto need not have estoppel effect. But, if your claim includes limitations as to what your invention can not do, then it surely will have estoppel effect. Because, if it is explicitly claimed that a certain invention can not do a function A and afterwards an amendment is done to claim list to include that the invention can perform the function A, that will face estoppel.