AFAIK, summary LSA generation by non-backbone ABR is/was the default JUNOS behaviour, please see "OSPF and ISIS: Choosing an IGP for Large-Scale networks" book by Jeff Doyle ISBN 0-321-16879-8 chapter 7.3 pages 224-228.
I haven't verified yet if this is still the case with newest JUNOS, so YMMV.
Yes Summery LSA can be generate by non backbone ABR and in your example area 1 summery can reach area 2 as i have tested in the lab and found that summery LSA from R2 to R5 with out backbone area. following senerio i tested...
OSPF was originally designed to Separate linkstate databases are maintained for each area, with area 0 acting as a sort of interconnect, but it's possible to have area-based routing in general without such a 'backbone area'. IS-IS, for example, has no such restriction.
Also, while the required area 0 might be a hurtle, you can create virtual links with transit areas to get around that restriction in a lot of cases.The requirement to have area 0, and the requirement to plan everything else around it, is one of my bigger peeves with OSPF in general. Physically there are better ways to organize routing for a network backbone.
Virtual link only requried if backbone area is configured, because in the presence of the backbone area summery LSAs only exchange through backbone. e.g.