Re: Installing gutters AFTER installing the drip edge
The gutter needs to be behind the drip edge and there is no harm in fastening through the drip edge into the gutter and into the fascia. If the roof is properly installed, then the drip edge acts only as a safety back-up because the roofing material sheds the water into the gutter, the drip edge just makes sure that water can't back travel under the shingles or if it gets to the roofing felt, it still gets to the gutter and not down the fascia.
When you call for quotes on seamless gutters, make sure the estimator and installers know that the gutter is to be retrofitted and installed behind the drip edge, then check their work to make sure you've gotten what you're paying for.
I suffer from CDO ... Its like OCD, but in alphabetical order, LIKE IT SHOULD BE!!!