I'm so sorry you are struggling with her slow growth. It does sound like getting her evaluated would be a good idea. Many kids improve a lot with therapy. Getting new clothes sounds like a good plan - just to give you and her a little boost. Just because they still fit does not meant hat she has to wear them.
FYI - as car seats go the bucket is no safer than a convertible - as long as the child fits the seat. Some babies start out rear facing in a convertible seat. If you want her in a convertible seat, then by all means put her in one. Keep in mind though that it is safer to rear face until at least the age of 2 (the new AAP recommendation.)
http://aapnews.aappublications.org/cgi/content/full/30/4/12-a
Also, keep in mind that in many states the law requires a minimum of 1 year old and 20 pound before forward facing. Many car seat as well have requirements of at least 22 pounds and some also have height requirements for forward facing.
