Can wearing your last invisalign tray for 7 weeks until retainers arrive cause treatment less effective?

Given you have worn your aligners for the recommended time (22hrs over 14days) and they have not cracked or broken since, the effectiveness of your invisalign treatment should not be compromised.  If you extend your wear on any one aligner they act to 'retain' your teeth in the current position. They are designed to be quite durable and brushing them won't result in them becoming 'ineffective'.It is advisable that you continue on wearing your aligner full time up until they can be replaced with retainers to avoid any possibility of 'relapse' or tooth movement during this waiting period.Goodluck! 

The only harm that might occur is that the trays may fracture however, even if that happens, you can wear the broken pieces until you can see your orthodontist a.s.a.p. if you have kept your previous tray you may also use it as well.