No mailing a 2018 now should not affect your 2019 return.  When you go to efile 2019 you will need to enter 0 for the 2018 AGI since you didn't file it on time.  You can go ahead and efile 2019 (even before they get 2018).

Yes it will take awhile for the IRS to open all the mail.   They just started opening the mail that's been stacking up in trailers for months.  So you will have to wait for any refund.

See this IRS page for the latest information

https://www.irs.gov/newsroom/irs-operations-during-covid-19-mission-critical-functions-continue

Prior year returns can only be printed and mailed. When you mail a tax return, you need to attach any documents showing tax withheld, such as your W-2’s or any 1099’s. Use a mailing service that will track it, such as UPS or certified mail so you will know the IRS received the return. And don't forget state.

Important! Mail Federal and State in separate envelopes and mail each year in separate envelopes.