Is it possible to file amendments for the descriptions/drawings at the same time as entering US national phase?
Preliminary amendments can be filed when a PCT application enters the national phase in the US. This is apparent from 37 CFR 1.115, which provides:
(a) A preliminary amendment is an amendment that is received in the Office (§ 1.6 ) on or before the mail date of the first Office action under § 1.104.
(b)…(3) A preliminary amendment will not be disapproved under (b)(2) of this section if it is filed no later than: …
(iii) Three months from the date the national stage is entered as set forth in § 1.491 in an international application.
The types of amendments are governed by 37 CFR 1.121. In particular, 37 CFR 1.121(b) makes clear that the description can be amended and 37 CFR 1.121(d) makes clear that the drawings can be amended.
There are limits on the amendments. Notably, 37 CFR 1.121(f) provides:
No amendment may introduce new matter into the disclosure of an application.