Gatwick to Heathrow

Also- there is a connection with an 1hr 10 min turnover at Heathrow, all in Terminal 5. Can that be done?

Go with the 1hr 10 min turnover at Heathrow. Three hours between flights isn't enough time to get from Gatwick to Heathrow. Even a 6 hour layover at Heathrow is better than schlepping from Gatwick to Heathrow.

Also- there is a connection with an 1hr 10 min turnover at Heathrow, all in Terminal 5. Can that be done?

Is this a single booking or are you stitching together separate bookings? If the former its tight but the airline seemingly thinks it can be done. If the latter, I see wailing and gnashing of teeth.

A 3 hr layover and airport change is an engraved invitation for disaster

That's my $0.02

British Air sells the Venice to San Diego flight with the 1hr 10 minute layover in Heathrow. We will have all carry-on luggage.

If BA sell a 70 minute same airport connection as one ticket, if for some reason you miss the connection (that is not your fault, like stopping to get a meal in the airport) they have to get you home on the next one at no extra cost.

Perfectly practical to do a T5/T5 transfer in this time on a single ticket.

If you arrive at a satellite terminal (B/C) and you connecting flight departs from one of these clear security in the satellite rather than the main building (A) to save time.

Don't even consider the Gatwick to Heathrow option. You'll likely end up having a stroke with all the stress and getting to see the National Health Service.

Even if you're at Heathrow for six hours, the pricing there is the same as in London, so you're not going to be paying exorbitant prices for food, drinks, or shopping.

I also recommend against trying to transfer from Heathrow to Gatwick in that short time.

One of the main bus services between the LHR and LGW is National Express. Its website shows transport time of 1 hr. 20 min. Even if your airline has a dedicated bus transfer for connecting passenger, I wouldn't risk it or want the stress of cutting timing too close.