Can I do the EASA part 66 theory exams before starting practical training?
I know very little about EASA regulations, but it looks like your first question is answered in EASA's Part-66 FAQ:


  I have completed my EASA Part-66 modules for B1 and I have passed all the exams, but I still lack experience to get my licence. Is there
  a time limit to get the licence ? Will the certificate expire in a few
  years if I do not get the experience?
  
  According to Commission Regulation (EU) No 1149/2011 of 21 October
  2011 (amending Regulation (EC) No 2042/2003), the basic examinations
  shall be passed and experience shall be acquired within the ten years
  preceding the application for an aircraft basic licence. The new
  regulation also states that for the purpose of time limits related to
  basic knowledge examinations, basic experience acquired before the
  Regulation applies, the origin of time shall be the date by which this
  Regulation applies, which is 01/08/2012 (which means until
  31/07/2022).


I couldn't find any clear answer for your second question, but another FAQ item might be relevant:


  Where do I gain the required basic maintenance experience? Is it mandatory to gain the required maintenance experience in an EASA
  approved Part-145 organisation?
  
  According to the AMC 66.A.30(a)(4), aircraft maintenance experience
  gained within different types of maintenance organisations (under
  Part-145, M.A. Subpart F, FAR-145, etc.) or under the supervision of
  independent certifying staff may be accepted by the competent
  authorities. This means that the aircraft maintenance experience may
  be accepted by the competent authority when such maintenance is
  performed in a maintenance organisation which does not necessarily
  hold an EASA Part-145 approval. However, it is on the competent
  authority to evaluate whether this experience is acceptable.
  
  Consequently, please contact the competent authority where you intend
  to apply for a licence, in order to check whether the basic experience
  would be acceptable, with detailed information on the type of
  aircraft, its operation and the nature of the work.


In other words, ask the authority in your country.