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The ~得る construction already implies possibility, so でき得る sounds redundant to me, like "if it's possible to be possible" (unless you were using できる to mean that something is finished, but your examples aren't).
Another possibility would be ～ものなら to indicate that achieving what you want might be difficult.
できれば、独立して事業を始めたい。 → Just a regular "if I can"
できるものなら、．．． → "If I really could (but I think it would improbable), ..."