Is a cIearance only valid once it's accepted?
Interesting question - although not FAA, I hoped the UK's CAP 413 would hold some insight as it tends to follow ICAO rules:

https://publicapps.caa.co.uk/docs/33/CAP413%20MAY16.2.pdf

Page 25 is the relevant chapter, and while it doesn't outright say the clearance is not valid it is very strong in its readback requirements. The following language applies to runway entry/take off/landing:


  The stringency of the read back requirement is directly related to the
  possible seriousness of a misunderstanding in the transmission and
  receipt of ATC clearance and instructions. ATC route clearances shall
  always be read back unless otherwise authorised by the appropriate
  ATS authority 
  
  The ATS messages listed below are to be read back in full by the pilot/
  driver. If a readback is not received the pilot/driver will be asked to do
  so. Similarly, the pilot/ driver is expected to request that instructions are
  repeated or clarified if any are not fully understood.
  
  [Lists the relevant activies, including runway entry]


It also points to ICAO Doc 4444:

https://ops.group/blog/wp-content/uploads/2017/03/ICAO-Doc4444-Pans-Atm-16thEdition-2016-OPSGROUP.pdf

With similarly direct wording:


  The flight crew shall read back to the air traffic controller safety-related parts of ATC clearances and
  instructions which are transmitted by voice. The following items shall always be read back:
  
  a) ATC route clearances;
  
  b) clearances and instructions to enter, land on, take off from, hold short of, cross, taxi and backtrack on any
  runway; and
  
  c) runway-in-use, altimeter settings, SSR codes, level instructions, heading and speed instructions and, whether
  issued by the controller or contained in automatic terminal information service (ATIS) broadcasts, transition
  levels.


It seems clear to me that ICAO, EASA and CAA standpoint is that readbacks are completely mandatory, and the clearance has not been fully delivered until the readback is completed. The readback is a controller final opportunity to correct a potentially critical misunderstanding or communication and it seems to me that proceeding without reading back, and waiting for any call of "Negative" is negligent.