rodandmegs,
The ladies are doing a great job of helping you talk through your options. Based on what you've shared thus far, and about your desire not to do too many guides, I do think your son could be placed in Bigger Hearts with Extensions. I would recommend that he do Rod and Staff English 4, instead of English 5, and do a lesson daily 4 days a week. I would also recommend that he do DITHR Level 4/5 Student Book (with the Teacher's Guide for you) along with the 4/5 Boy Set linked here:
http://www.heartofdakota.com/DITHOR45-boys-pack.php You could do DITHR 3 times weekly in order to lighten your load.
As far as math goes, it would be wise to give him the Singapore Placement Test U.S. Edition linked here:
http://www.singaporemath.com/Placement_Test_s/86.htm
Begin with the test for 3A and see if he passes. Make sure not to aid, help, or explain in any way, or the results will not be accurate. It would not be uncommon for a 5th grader who is new to Singapore to have to start back somewhere between 3A and 4B, simply due to the differing scope and sequence. I wouldn't start a 5th grader who is new to Singapore in 5A/5B as it will assume that he has had the Singapore method and foundation laid in 3B on up to guide and help him.
You can return your English 5 and Singapore 5A/5B to Heart of Dakota and exchange them for the texts you need instead.

We want to be sure that you have what you need to have a successful school year!
As far as your older daughter goes, if my experience with kiddos her age is correct, she is deeply desiring independence due to her age and increasing maturity level. She probably feels that CLE for her specific grade level will insure that she will not be combined with her younger sibling. If she felt sure that doing Creation to Christ also meant she would not be combined, she would likely be on board with that as well. I would be interested to hear where she places on the first page only of the placement chart when you consider her all by herself (no combining scenarios in mind). It is possible that she may fit in Resurrection to Reformation instead. If you get a chance to share a bit more about her reading, math, grammar, writing, independence level, and ability to read her own materials that would be great! Link:
http://www.heartofdakota.com/placing-your-child.php
Blessings,
Carrie